Read-only mode is a setting an admin can enable to prevent writes to

the database. This can be used when the site is under maintenence or

being upgraded. This commit disables most UI features which would

normally affect Review Board, but under read-only mode, does not.

Disabling some features such as login and redirection when attempting

to view a read-only restricted page is contained in the next commit.
This commit contains changes to the datagrid so changes to it does not

change profile settings, navbar (hide My Account and New Review Request

buttons), ETag, ReviewRequestEditor(including actions, comments, and

the editor itself), 
Other changes include adding read-only mode setting to the context

processor and adding a utility file containing the logic to determine

whether a user is in read-only mode or not.

Check that caching of read-only mode is done correctly by changing

modes and refreshing pages
Check that profile doesn't save in datagrid when in read-only mode by

ensuring code doesn't reach the save method
Check that the review request actions available are appropriately

changed when in read-only mode
Check that comments can't be added or replied to in read-only mode
Check that review requests can not be modified in read-only mode
Check that modifying review requests (such as bugs or branch) when in

read-only mode while the site in browser is still old displayed an

appropriate error message
Check that modifying anything to the webapi doesn't send a request at

all when in read-only mode
Check that trying to add a comment through clicking on a line in diff

displays appropriate warning about read-only mode
Check that clicking on the Star does nothing in read-only mode
Check that the above does not occur for admins