fix(slack): Escape assignee dropdown input

[isabellaenriquez]

Fixes SENTRY-2MBQ

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.11%. Comparing base (cd44047) to head (56e036b). Report is 4 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #71105       +/-   ##
===========================================
+ Coverage   56.14%   78.11%   +21.96%     
===========================================
  Files        6625     6588       -37     
  Lines      296381   294279     -2102     
  Branches    51053    50783      -270     
===========================================
+ Hits       166416   229881    +63465     
+ Misses     125367    58095    -67272     
- Partials     4598     6303     +1705     

Files	Coverage Δ
...sentry/integrations/slack/webhooks/options_load.py	96.15% <100.00%> (+44.15%)	:arrow_up:

... and 3456 files with indirect coverage changes

[getsantry[bot]]

This pull request has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you add the label WIP, I will leave it alone unless WIP is removed ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀