feat: Add different comment message for 1st time pulls

[scott-codecov]

codecov/engineering-team#389

Companion PR to codecov/codecov-api#108

If a pull is the 1st for a repo then we want to show different copy in the PR comment.

[codecov]

Codecov Report

Merging #89 (bca9aee) into main (c828650) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #89   +/-   ##
=======================================
  Coverage   98.47%   98.47%           
=======================================
  Files         362      362           
  Lines       26519    26549   +30     
=======================================
+ Hits        26115    26145   +30     
  Misses        404      404           

Flag	Coverage Δ
integration	98.44% <100.00%> (+<0.01%)	⬆️
latest-uploader-overall	98.44% <100.00%> (+<0.01%)	⬆️
unit	98.44% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
NonTestCode	97.13% <100.00%> (+<0.01%)	⬆️
OutsideTasks	98.24% <100.00%> (+<0.01%)	⬆️

Files Changed	Coverage Δ
database/models/core.py	96.98% <100.00%> (+0.05%)	⬆️
...ervices/notification/notifiers/comment/__init__.py	99.43% <100.00%> (+0.01%)	⬆️
...cation/notifiers/tests/integration/test_comment.py	92.34% <100.00%> (+0.17%)	⬆️
.../notification/notifiers/tests/unit/test_comment.py	100.00% <100.00%> (ø)
tasks/tests/integration/test_notify_task.py	100.00% <100.00%> (ø)

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[giovanni-guidini]

looks good in general. I like the welcome message.
Just left a couple of potentially silly questions, but it's honestly fine as is.