Fix broken links to glviewer and attachments

Hardcoded relative links are replaced with calls to url_for to the view
endpoint.
Merge branch 'fix_glviewer_endpoint'

* fix_glviewer_endpoint:
  Fix broken links to glviewer and attachments
Fix for 512 tile size (tiff)
Merge branch 'flip'

* flip:
  Fix for 512 tile size (tiff)
Add support for login via LinkedIn's OAuth2

The LinkedIn icon is from: https://github.com/intridea/authbuttons

Usage of the LinkedIn logo here should be fair use.

Closes #31.
Merge branch 'login_linkedin'

* login_linkedin:
  Add support for login via LinkedIn's OAuth2
Add a missing package to the requirements.txt file
Merge branch 'missing_requirement'

* missing_requirement:
  Add a missing package to the requirements.txt file
Fix a performance regression when displaying the session list

Referencing 'session.database' inside the innermost loop caused additional
Mongo queries. Using 'role.db', which is already cached, improves the
performance of large page loads.
Merge branch 'sessions_performance'

* sessions_performance:
  Fix a performance regression when displaying the session list
Update Apache WSGI to suggest setting an path to the site config file

After extensive research, I can't find any way to change the environment
of the WSGI daemon before it imports the WSGI site file for the first
time. Apache's "SetEnv" directive only applies to the requests that are
provided to the site.

An alternative location for placing site-specific configuration files
outside of the code would be Flask's instance directory. While potentially
more appropriate for an installed app, it is less flexible than the
environment variable approach.
Merge branch 'apache_site_config'

* apache_site_config:
  Update Apache WSGI to suggest setting an path to the site config file
Add default settings for LinkedIn login
Merge branch 'login_linkedin'

* login_linkedin:
  Add default settings for LinkedIn login
Rename "about" page to "status", change "about" link to Kitware page
Emphasize the navbar's login button more
Fix bug to ensure "sessions" toolbar button is highlighted when active
Remove a stray character
Rename "Slide Atlas" -> "SlideAtlas" and make headings more consistant
Only display Admin button if logged in

This should eventually be changed to only display if the current user has
administrative privileges.