clean up style sheet

- remove unused colors / styles
- provide and use standard bioc colors
- standardize (incomplete) use of colors
style main navbar
move user login / faq bar right of image
format tags smaller, without embedded 'x', more columns
move search bar
lower-case My: tags
gray background on menu hover
separate tag from count on tags page
reduce 'Recent' sidebar information
search box in first line

- hack: place visible-sm, -xs boxes below so 'Live search:...' appears
  on main page
- hack: comment out search_bar.html div, so space displays but no box
right sidebar visible-md
better alignment
right-align all user status info
don't highlight user info when message selected
sidebar visible in large mode, too
float tags on main page

- short signatures align with tag
remove need for custom.py
hover green over ASK
conditional setting of EMAIL_BACKEND
Merge branch 'style' of https://github.com/Bioconductor/support.bioconductor.org into style