Flatten authors and contributors array (#1214)

The authors and contributors of exercises are stored in an array in the exercises' `.meta/config.json` files.
We used to have separate fields for the GitHub- and Exercism username of the user.
As we're only using the GitHub username, we've flatten the `authors` and `contributors` array to an array of strings representing the GitHub usernames.

See https://github.com/exercism/docs/pull/90/files

Author: ErikSchierboom

exercises/concept/assembly-line/.meta/config.json

@@ -1,14 +1,8 @@
 {
   "blurb": "Learn about numbers while working on an assembly line for cars.",
   "authors": [
-    {
-      "github_username": "LewisClement",
-      "exercism_username": "LewisClement"
-    },
-    {
-      "github_username": "efx",
-      "exercism_username": "efx"
-    }
+    "LewisClement",
+    "efx"
   ],
   "files": {
     "solution": [

exercises/concept/csv-builder/.meta/config.json

@@ -1,18 +1,9 @@
 {
   "blurb": "TODO: add blurb for csv-builder exercise",
   "authors": [
-    {
-      "github_username": "gilescope",
-      "exercism_username": "gilescope"
-    },
-    {
-      "github_username": "coriolinus",
-      "exercism_username": "coriolinus"
-    },
-    {
-      "github_username": "efx",
-      "exercism_username": "efx"
-    }
+    "gilescope",
+    "coriolinus",
+    "efx"
   ],
   "files": {
     "solution": [

exercises/concept/entry-api/.meta/config.json

@@ -1,10 +1,7 @@
 {
   "blurb": "TODO: add blurb for entry-api exercise",
   "authors": [
-    {
-      "github_username": "seanchen1991",
-      "exercism_username": "seanchen1991"
-    }
+    "seanchen1991"
   ],
   "files": {
     "solution": [

exercises/concept/options/.meta/config.json

@@ -1,14 +1,8 @@
 {
   "blurb": "TODO: add blurb for options exercise",
   "authors": [
-    {
-      "github_username": "seanchen1991",
-      "exercism_username": "seanchen1991"
-    },
-    {
-      "github_username": "coriolinus",
-      "exercism_username": "coriolinus"
-    }
+    "seanchen1991",
+    "coriolinus"
   ],
   "files": {
     "solution": [

exercises/concept/semi-structured-logs/.meta/config.json

@@ -1,10 +1,7 @@
 {
   "blurb": "Learn enums while building a logging utility.",
   "authors": [
-    {
-      "github_username": "efx",
-      "exercism_username": "efx"
-    }
+    "efx"
   ],
   "files": {
     "solution": [

exercises/concept/structs/.meta/config.json

@@ -1,10 +1,7 @@
 {
   "blurb": "TODO: add blurb for structs exercise",
   "authors": [
-    {
-      "github_username": "seanchen1991",
-      "exercism_username": "seanchen1991"
-    }
+    "seanchen1991"
   ],
   "files": {
     "solution": [

exercises/concept/tuples/.meta/config.json

@@ -1,10 +1,7 @@
 {
   "blurb": "TODO: add blurb for tuples exercise",
   "authors": [
-    {
-      "github_username": "coriolinus",
-      "exercism_username": "coriolinus"
-    }
+    "coriolinus"
   ],
   "files": {
     "solution": [

exercises/concept/vec-macro/.meta/config.json

@@ -1,14 +1,8 @@
 {
   "blurb": "TODO: add blurb for vec-macro exercise",
   "authors": [
-    {
-      "github_username": "efx",
-      "exercism_username": "efx"
-    },
-    {
-      "github_username": "coriolinus",
-      "exercism_username": "coriolinus"
-    }
+    "efx",
+    "coriolinus"
   ],
   "files": {
     "solution": [

exercises/practice/accumulate/.meta/config.json

@@ -1,9 +1,7 @@
 {
   "blurb": "Implement the `accumulate` operation, which, given a collection and an operation to perform on each element of the collection, returns a new collection containing the result of applying that operation to each element of the input collection.",
   "authors": [
-    {
-      "github_username": "sacherjj"
-    }
+    "sacherjj"
   ],
   "files": {
     "solution": [

exercises/practice/acronym/.meta/config.json

@@ -1,9 +1,7 @@
 {
   "blurb": "Convert a long phrase to its acronym",
   "authors": [
-    {
-      "github_username": "gregcline"
-    }
+    "gregcline"
   ],
   "files": {
     "solution": [