Lit bulk merge tag conflicts

[caseyhans]

Adds capability to bulk merge user tags for all references in an assessment, accessed under the Actions menu on the conflict resolution page.

• Adds unique constraint to ReferenceTag and UserReferenceTag through models; this migration may take time on larger databases
• Fix button bug for 'Full Text' button in reference component
• Modifies hyperlink for the change log list from 'Change logs' to 'View change logs' for clarity
• Alters the change log 'Log' button color to primary so it looks like a button on EPA flavor (btn-outline is broken for EPA flavor, but we have a custom outline-btn class that works)

[shapiromatron]

Wow, this was impressive. Thank you for teaching me about the resolution options for bulk_create and bulk_update; these are really exciting to think about.

I had one comment about a maybe redundant if statement, and then lots of minor edits and changes for you to review. If you're ok w/ the my commits (I tried to make them small and atomic) and we resolve the comment, ok to merge:

• 63666ce minor docstring updates (25 seconds ago) {Andy Shapiro}
• fac8dcc write a flatten helper method (33 seconds ago) {Andy Shapiro}
• a1a4fdf use TZ aware datetime (5 minutes ago) {Andy Shapiro}
• 5cef18b use kw names in func calls (23 minutes ago) {Andy Shapiro}
• 82ac464 add docstring (30 minutes ago) {Andy Shapiro}
• 7b26316 use boolean check instead of equality in template (31 minutes ago) {Andy Shapiro}
• 15b8e67 use set comparisons instead of numpy (31 minutes ago) {Andy Shapiro}
• c7b23c9 revert this change (33 minutes ago) {Andy Shapiro}
• 0a462e8 for consistency, make all "Change logs" -> "View change logs" (3 hours ago) {Andy Shapiro}
• a212cc2 disable cache_key field (6 weeks ago) {Andy Shapiro}
• f3a525d use our django modal template (6 weeks ago) {Andy Shapiro}
• 734795a remove api endpoint; support form view for now (6 weeks ago) {Andy Shapiro}
• 67f6715 DRY permissions check (6 weeks ago) {Andy Shapiro}
• 389deff apply constraint cleanup for both models (6 weeks ago) {Andy Shapiro}
• ee345c3 simplify css (6 weeks ago) {Andy Shapiro}