Depending on the operating system and therefore the system font
the back icon being a unicode arrow means it'll render inconsistently,
sometimes I've seen it looking really odd.
Instead make use of the font awesome arrow so that'll it render consistently
no matter ths system font.
ComposeBox already specified autoFocus for the ComposeInput.
However, this didn't work because the dialog was disabled when the ComposeInput code tried to focus the textarea.
To fix this, tweak A11yDialog to look for the autofocus attribute when determining what to focus.
This is consistent with the behavior for native HTML dialogs.
Then, have ComposeInput set this attribute if it's in a dialog.
Previously, if focus was on an element inside a toot instead of the toot itself (e.g. moving to a toot and pressing tab), keyboard commands acted as if no toot was active.
In particular, this meant that the arrow keys scrolled to the first visible toot.