Add KaTex math rendering

Add KaTex javascript, CSS, and fonts to vendor folder.

Create a renderMath() function that parses sanitized markdown for math as denoted by '$$'

Modify ctrl.$render to first sanitize the markdown and then renderMath. Newly rendered
math is then trusted as HTML becuase otherwise ngSanatize removes crucial style attributes
that leave the math unreadable.
Add MathJax fallback

If KaTex math rendering throws an error, load MathJax from the CDN and render
the math with MathJax.
Take renderMath() and MathJaxFallback off of scope
Modify assets.yaml to put KaTex.min.js directly into the bundle
Specify MathJax Typeset call with output element
Breakout loading MathJax into separate function
Add spaces between items in list on line 311
Break out loadMathJax() function to Module level

Delete console.log error
Move docstring comment next to directive definition

Remove unneeded math variable

Fix typo in forgetting to remove an instance of MathJaxFallback from the scope
Implement try/catch in the render function
Patch for previous commit
Give $timeout third parameter

Kill if !MathJax?
Fix bug

Multiple instances of Math in one annotation weren't rendering well. Put return in front of renderMath() recursive call.
Merge pull request #1532 from hypothesis/math-support

Add math support
Hide highlight icon when editing an annotation
Merge pull request #1535 from hypothesis/no-highlighter-when-editing

Hide highlight icon when editing an annotation
Tooltip on heatmap tabs

Close #867
Show spinner on account and auth form submits

Close #1213
Fix failing auth tests
Don't resolve the updater in the promise store

Besides potentially delaying the websocket connection unnecessarily,
this was causing a subtle race condition wherein the stream would
miss a digest and appear blank. Reasons are unclear, and may hide
dragons.