Encode HTML on edit

[raineorshine]

Currently, basic HTML formatting tags (b, i, strong, em) are allowed to be typed into a thought and rendered as HTML. All other tags are stripped out. In practice, this has not been very useful, and has gotten in the way of typing literal representations of HTML.

Instead, encode the HTML as character entities so it is rendered literally.

Handling of HTML in pasted content depends on the content-type:

• When the pasted content-type is text/html, we should render the formatting tags as HTML as done currently.
• When the pasted content content-type is text/plain but contains HTML (i.e. htmlText is empty and isHTML is true), we should escape the HTML and render character entities.

Three cases:

• Type HTML: Escape HTML and render literal character entities (changed behavior)
• Paste text/plain with HTML: Escape HTML and render literal character entities (changed behavior)
• Paste text/html: Render formatting tags (current behavior)

It is very likely that fixing one could break another if you are not careful. Therefore, it is recommended that you test all three cases after fixing one to avoid a regression. We currently do not have testing logic to mock pasting text with different content-types. You could try to mock e.clipboardData.getData, or it is acceptable to just do manual testing.

1. Type HTML

Steps to Reproduce

1. Create a new thought
2. Type hello <b>world</b>
3. Hit escape or move the cursor

Current Behavior

The HTML is NOT escaped. The thought gets rendered as "hello world".

Expected Behavior

Typed angled brackets should be escaped, i.e. converted to character entities.

The thought's value should be hello <b>world</b>

Which should get rendered literally as hello <b>world</b>

2. Paste text/plain

Steps to reproduce

1. Type hello <b>world</b> in your text editor.
2. Copy hello <b>world</b> from your text editor. This will ensure text/html is not added to the clipboard.
3. Paste into a new thought

Current Behavior

The HTML is NOT escaped. The thought gets rendered as "hello world".

Expected Behavior

Same as typing text/plain.

The thought's value should be hello <b>world</b>

Which should get rendered literally as hello <b>world</b>

3. Paste text/html

Steps to Reproduce

Copy and paste this text into a new thought: hello world
(Copying the rendered text from the browser will copy the HTML into the text/html part of the clipboard.)

Current Behavior

The HTML is not escaped. The thought gets rendered as "hello world".

Expected Behavior

The current behavior should not change.

Testing

Add tests for each case.

I'm not sure what testing level to go for here: importText reducer or RTL tests, or a combination. Prefer reducer tests when possible.