Closes #64

Closes #69

Fixes #23
Closes #50

Closes #66

Closes #63

- Search controller (functionally the same, presents results on top of
explore menu)
- Add bookmarks screen

See #63

Also, copy the state between screens, so e.g. expanding a status in the
timeline and then opening that conversation already has that status
expanded.

This intentionally doesn't store the sensitive attachment visibility
state, since showing text when not necessary is less dangerous than for
images. (Possibly a preference for this in the future?)

Closes #55

Closes #54

This needs a refactor once StatusTableViewCell and
ConversationMainStatusTableViewCell are merged. See #54

viewWillDisappear method and send preferences changed notification

Workaround for #36

instead of replacing font entirely.

Fixes #51

Closes #31

Closes #46

Closes #45

Closes #35

Closes #28

#28

Primarily in preparation for timeline position persistence and split
timelines

Use AutoLayout + stack views instead of manual frames, fixes attachments
sometimes not using the whole width

Closes #18

Italics, bold, strikethrough, monospace, and (un)ordered lists