Add API support for the rich_text field on several resources.
rich_text was previously the default for any new objects, and clients

were expected to send text in valid Markdown. This posed a problem for

Review Bot, RBTools, and probably a lot of scripts used in production,

which would suddenly have to know about Markdown.
This change gives clients more control. Now, by default, rich text is an

opt-in thing. It's disabled on new objects by default, but callers can

pass rich_text=true in API calls to indicate that the text fields for

that object should be rich text.
For resources with multiple affected fields, setting rich_text to true

will set the provided text as-is, and escape any text fields not set

during that API call, ensuring their contents won't accidentally be

interpreted as Markdown.
Likewise, if setting rich_text=false, any unspecified text fields will

have any escaping removed, leaving just the text content (meaning that

**This** is a \*test\* will turn into **This** is a *test*).
The web UI always sets rich_text=true in calls. It also ensures that the

text coming in is Markdown, even if it's not stored as Markdown on the

server. That is, if rbt post provides a description for a review

request, then server-side, rich_text will be false, but the web UI will

then escape it when displaying. This ensures that the user will see

Markdown-formatted text and can act appropriately when they go to save

(which will formally set rich_text=true on the server).
Unit tests for the API and UI have been added.

Python and JavaScript unit tests pass.
Tested the following scenarios:
Review requests

Posted a change for review, with a description that would be otherwise interpreted as Markdown.

  Saw that it escaped properly for display, but was internally stored with the original content

  and rich_text=false.
Edited the Description and Testing Done fields individually when a review request had

rich_text=false. The other fields properly escaped, but the new content went through.

  The server had rich_text=true after the save.
Did the same test with a change description.

Reviews

Tested creating a review and saw that it was in Markdown by default.
Took an old review with rich_text=false and edited it. The original text was properly escaped.

  Saving it set rich_text=true.

Comments

Created a comment for a new review, and saw that it was Markdown by default. Repeated with a new

  comment on an existing review. Published these and saw my Markdown was rendered.
Took an old comment with rich_text=false and edited it. The original text was properly escaped.

  Saving it set rich_text=true.

Replies

Made a new reply to a comment and added Markdown. It properly rendered, before and after a reload.

  Saw in the database that rich_text=true.
Edited an older, unpublished reply with rich_text=false. The text was escaped in the input box.

  Saved and it saw that rich_text=true, and saw it render properly.
Repeated the tests with a reply to a review's body text.