AO3-6831 Rename the Underage warning tag

[sarken]

Issue

https://otwarchive.atlassian.net/browse/AO3-6831

Purpose

• Adds two rake tasks:
  • bundle exec rake After:add_suffix_to_underage_sex_tag to rename the Underage Sex freeform to Underage Sex - Freeform
  • bundle exec rake After:rename_underage_warning to rename the existing Underage warning to Underage Sex
• Updates tests, configuration, and documentation to reflect the rename

Testing Instructions

None needed, already tested.

References

The TOS update itself will contain this change:

3. https://ao3.org/first_login_help (

   otwarchive/app/views/home/first_login_help.html.erb

   Lines 125 to 126 in 1bdb444

   	Depictions Of Violence", "Major Character Death", "Rape/Non-Con", and
   	"Underage". When posting a work, you have the option to explicitly select

   )
   "and "Underage"" should be "and "Underage Sex""

This is because the TOS update internationalizes the text from that file, so there was no clean way to include the change here.

Credit

Sarken, she/her