Fixed admin tab selection

Admin tab selection was relying on a specific url deployment with a set
number of path components. Now it simply looks at the last component
which is reliable
View to email select users, groups or all users

Added new view with email icon to send email to omero users
Makes use of django's email capabilities until omero email features are
available
Email capabilities enabled only if email settings are set
Send webadmin email as html as well as plain text
Fixed display of no email settings

Fixed the admin tab highlight when the email disabled page is shown
Removed unused blocks form noemail template
Remove extra whitespace in email template