[ticket/11040] Search subject and text together

Instead of searching for post_subject and post_text seperately and then
OR them in where clause we use concatenation of the two columns and search

PHPBB3-11040
[ticket/11040] Add post_content index

In addition to post_subject and post_title indexes add a post_content
which is concatenation of both the columns.

PHPBB3-11040
[ticket/11040] Remove postgres extra indexes

Remove post_text index as post_content index is sufficient to search post
text.

PHPBB3-11040
[ticket/11040] Add test cases for searching subject and post content together

PHPBB3-11040
[ticket/11040] Swap post_text and post_subject for post_content index

Provides better performance as per PostgreSQL Docs.

PHPBB3-11040
[ticket/11040] Add methods to delete post and topic in functional tests

PHPBB3-11040
[ticket/11040] Delete the functional test topic to avoid conflicts

PHPBB3-11040
[ticket/11040] Add migration to drop postgres search indexes

PHPBB3-11040
[ticket/11040] Use hard delete in delete_topic

PHPBB3-11040
[ticket/11040] Use unique text for the test post added

PHPBB3-11040
[ticket/11040] Topic is deleted if test is skipped

PHPBB3-11040