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Allow inputing multiple channels/users in integrations; comma-separated

Closes #3121

* #2941 prevent default on enter key in search channel/direct message view

* CR fix

* unused variable

This fixes problems we had with file uploads on Microsft Edge

* cs translations fixed

* merges

* fixes

Improves message quoting

- Use guest name or username as room and subscription name
- Save guest _id on rooms to best lookup
- Some UI fixes

Show correct menus on mobile

Show correct menus on mobile

* wait until user is logged-in to add message listener

* use Tracker rather than the onLogin event

Don't render empty katex

Updated katex to 0.6.0 and enabled $...$ and $$...$$ syntax

Replace empty links/clickable divs with <button/> (addresses #3045)

Added quote button to messages.