Fix a regression in HTML escaping for text area fields.

Text area fields were no longer escaping the HTML-unsafe characters
contained in the source text, either when dealing with plain text or
Markdown escaping.

We now escape the text in the base TextAreaField after normalizing it.
Add hooks for the dashboard and user page sidebar items.

These new hooks allow for plugging content into the dashboard and user
page sidebars, using the new sidebar item registration APIs. They
replace the old DashboardHook and UserPageSidebarHook.

There's technically also a DataGridSidebarItemsHook, but it's more of a
base class. Since there's only two datagrids with sidebars right now,
I'm leaving it undocumented until we start seeing a need to expose it.
Make a couple simple improvements to some datagrids.

This makes a couple changes to the "Submitters" and "Groups" datagrids. To
start, I've renamed "Submitters" to "Users". I've also switched the order of
"Users" and "Groups". Finally, I've renamed the "Pending Reviews" column to be
"Open Review Requests".

Testing done: Looked at the changed datagrids.

Reviewed at https://reviews.reviewboard.org/r/5417/