several changes to the sessions pages

- add a banner at the top of some pages
- extract the session display out into a shared view
- sort the sessions by name, with the current user's sessions at the top

TBH, these got a bit tangled together, so I can't really commit them separately
switch the github logo for a better one
add the twitter js as a bundle
ensure the twitter js is loaded
make more code use the standard formatting of sessions, including the tweet button
add the banner to the speakers page
remove the unused about page
tidy the external login error page
put the remove button next to the login
tweak the speakers page styles

 - make the icons smaller
 - add a separator between speakers