Add merge tags tool

[simonw]

Add a tool for merging two tags at /admin/merge-tags/.

• Add a view in blog/staff_views.py for handling the merge tags functionality with the staff_member_required decorator.
• Add a URL pattern in config/urls.py for /admin/merge-tags/.
• Create a form in templates/merge_tags.html with two input boxes for selecting tags using the /tags-autocomplete/ JSON endpoint.
• On POST, update quotations, entries, and blogmarks tagged with the loser tag, remove the loser tag, and add the winner tag.
• Delete the loser tag and create a PreviousTagName record pointing to the winner tag.
• Add unit tests for the merge tags feature in blog/tests.py.

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[simonw]

Direct assignment to the forward side of a many-to-many set is prohibited. Use tags.set() instead.