style.css: Substitute repeated colours with variables

Whenever possible (light and dark scheme use same variable name),
the element was removed from the dark block.
Add CSS for email fields
Improve dark theme style for text inputs
style.css: Add dark mode
Remove client from map on disconnect
Display error popup on FAIL
Add some space between error message and close button
Move dialog submit buttons to the right
Break long tag attributes on multiple lines

Makes that stuff more readable.
lib/irc: escape/unescape everything in tags

Without the greedy flag on the regexp, String.replace will just
replace the first match.

While at it, also make sure to convert to a string when formatting
tag values. This allows tag values to be e.g. numbers.
lib/irc: accept tags without value
Rename Connect/Join to ConnectForm/JoinForm
Cleanup compareBuffers
Trigger client error if server doesn't support SASL
Only switch buffer on connect if there's no active buffer
Fix whitespace issues
readme: add IRC discussion channel
readme: document query params
Remove unused DEFAULT_NETWORK constant
Fix overflowed dialogs
Add help section for commands