Sanitize string provided to to_tsvector

What does this MR do and why?

Closes #428428 (closed).

We sanitize full text queries for non-allowed characters, but we don't do this for generating the search vector itself.

When we supply non-allowed characters to to_tsvector, it can affect the resulting search vector.

Example: providing <gitlab> string to to_tsvector will result in an empty tsvector.

gitlabhq_production=# SELECT setweight(to_tsvector('english', '<gitlab>'), 'A');
 setweight
-----------

(1 row)

By sanitising the non-allowed characters out, it allows for words surrounded by these non-allowed characters to be included in the search vector

gitlabhq_development=# SELECT setweight(to_tsvector('english', ' gitlab '), 'A');
  setweight
-------------
 'gitlab':1A
(1 row)

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Pre-change:

1. Create an issue with the title the <rain> is falling down
2. Check the issue search data's search vector: Issue.where(title:'the <rain> is falling down').first.search_data.search_vector
   1. The search vector does not contain the word rain
3. Search for the term rain using Basic Search in the project
   1. There are no results in Issues

Post-change:

Assumes the issue already exists with the title as per pre-change steps.

1. Update the issues's search data: Issue.where(title:'the <rain> is falling down').first.update_search_data!
2. Check the issue search data's search vector: Issue.where(title:'the <rain> is falling down').first.search_data.search_vector
   1. The search vector includes the word rain
3. Search for the term rain using Basic Search in the project
   1. There is a result matching the issue

MR acceptance checklist

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