Bookmarks and siteUtil hardening

[bsclifton]

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• new isMoveAllowed (pulled out from moveSite)
• added unit test to ensure moveSite can't put an ancestor into a descendant
• about:bookmarks now calls isMoveAllowed before submitting action
• about:bookmarks allows folder drag onto bookmark; it gets parent folder ID and puts there
• fixed folder selection bug when searching on about:bookmarks (selection cleared during search)

Fixes #4766

Auditors: @bbondy

Test Plan:

1. Open about:bookmarks
2. Create two folders and also bookmark several items, under Bookmarks Toolbar
3. Click to select a folder which has bookmarks
4. Drag one of the folders and drop it on a bookmark
5. Folder should move into the folder that contains bookmark that was dropped onto
6. Making sure a folder you created is selected on the left (one that is NOT bookmarks toolbar or other bookmarks), do a search
7. Validate folder selection is cleared on left, in favor of "All Folders"
8. Drag one of the search results onto a folder on the left
9. Validate bookmark was moved

[bbondy]

lgtm