PNG size optimisations, fix icon paths in firefox app (and use our logo)

- optipng all the images!
- we're not using glyphicons-halflings.png (it's coming from a CDN)
- replace WeeChat logo with Glowing Bear logo in Firefox app & fix paths
Crunch the svg as  well
Layout fixes

- make buffer-/nicklist items that line up with the input bar clickable (z-index)
- add back padding for inputbar with nicklist on mobile
Improve mobile design
Adjust to actual mobile devices, not small browser windows
Fix scrolling on landing page (mobile)
Show timestamps on mobile by default

This is overriden by the setting
Turn notimestamp into a positive setting (showtimestamp)
Fix colours on mobile

@creesch introduced this change, but I'm not sure why
Make timestamps smaller and all the same colour on mobile

Different colours on mobile don't really work: either they're too similar to
distinguish (and then it just looks strange), or they're too dark to be visible.
Sans-serif on mobile, smaller margins, bigger nicks

@torhve: smaller margins! just click merge right now? ;)
Fix highlighting of channel upon selection in mobile firefox