Implement pinned messages + message deletion

[bates64]

@PullJosh did a design for tabs in the right sidebar; this is it in fruition!

• Contains some API spec changes: two new events and an unpin endpoint.
• You can delete/pin/unpin messages now with the UI (see #115, fixes #17)
• Note that the mentions tab is left blank for when #92 is finally complete, and the edit message icon does nothing when clicked; see #115.
• Includes / iterates on #254 - channels are now loaded from the oldest unread message
• Blocked by / includes #253. Merge that first. :ballot_box_with_check:

[towerofnix]

Pre-code-review notes:

Note - trying to view the "pins" tab while no channel is selected causes an error, because it fetches /api/channels/undefined/pins. Pins tab should probably be hidden while not actively in a channel? (And should switch to "users" tab when going back to a state with no channel active.)

Note - it's sorta hard to tell exactly what message my mouse is on top of, since the icons appear so far to the right. Maybe a dotted line from the message icons to the first line of the related message would help?

Note - on an authorized server, when I reloaded the page, there was an "authorization related error", but I'm 60% sure that doesn't have anything to do with this.

Note - flairs don't show up on pinned messages. I assume this is intentional? (It looks better without flairs imo.)

Note - the most recently pinned messages show up at the bottom of the pinned message list. IMO there should be a way to sort pinned messages - maybe even manually. Otherwise, they should probably be sorted by date. (Should the newest messages go on the top or the bottom?)

Note - the highlight from jumping to a pinned message doesn't look any different in the dark mode, but this is probably fine.

[bates64]

Hold on, just adding that dotted line thing you asked for.