Revamp/deprecate tag aliasing, migrate aliases to new tags

[jywarren]

We have 3 scenarios which relate tags to one another:

1. "incidental need to show 2 topics"
   - all good: https://publiclab.org/search?q=water+education, https://publiclab.org/search?q=water-education
   - https://booleanstrings.com/2018/03/08/the-full-list-of-google-advanced-search-operators/
2. "these are the same"
3. "this belongs under that"

Our current "tag aliasing" system is very fragile and has lots of cumbersome code, and is hard to maintain.

Proposal: migrate tag alias system to automated re-tagging system

• add preferred tag to all sub-tags (using a script)
• resubscribe people to the larger tag
• shut down aliasing, remove code
• real-time auto-tag additions (from a list) - break this out, as it's complex
  • source from main topics list (subtopics from footer) and related tags list (for search - Call for related search terms #4928 ) (synonyms) and auto-add these whenever a subtag is added by anyone

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Notes on different solutions to this issue:

1: https://publiclab.org/tag/legal+evidence+permits+regulations
- (ALL, or any content that has ALL those tags) - logical operators?
does this mean restricting results in this URL to content that only has all four of these tags?
- what's useful?
2: https://publiclab.org/wiki/combined-legal grid:
- [notes:legal+evidence+permits+regulations] (manually doing it on a wiki page)
useful?
3: tag aliasing: https://publiclab.org/tag/legal (where evidence+permits+regulations are aliased from it...)
- content tagged 'oil' already appears to be showing up on /tag/oil-and-gas, so i suppose this is aliased already?
- This is an old feature that doubles the work that the server has to do. Currently doubling the amount of work that goes into any feature.
4: manual re-tagging
- bad --> i would like to revisit this adjective.
5: Automated Re-tagging
- write a script that adds "oil-and-gas" to any "oil" or any "gas" post
6: real-time auto-tagging - at publish time

for 6, we can expand this:

• use a config file with tag/parent pairs instead of writing them inline as in Add additional related "subtags" upon adding new tags #9398
• source more of these from Call for related search terms #4928 and add them in

[ebarry]

This is great!