User directory search doesn't work when searching for user IDs.

Description

The spec states that the user directory search should be "performed case-insensitively on user IDs and display names preferably using a collation determined based upon the Accept-Language header provided in the request, if present".

The user directory works by performing a full text search on text vectors extracted from user IDs, for example:

synapse=# select * from user_directory_search limit 5;
            user_id            |                   vector                    
-------------------------------+---------------------------------------------
 @negritoofcourse:matrix.org   | 'matrix.org':2 'negritoofcours':1A,3B
 @freenode_jin1200:matrix.org  | 'freenod':1A 'jin1200':2A,4B 'matrix.org':3
 @freenode_Xenguy__:matrix.org | 'freenod':1A 'matrix.org':3 'xenguy':2A,4B
 @morethanabitoff:matrix.org   | 'matrix.org':2 'morethanabitoff':1A,3B
 @curtisthe:matrix.org         | 'curtisth':1A,3B 'matrix.org':2
(5 rows)

The query doesn't seem to take into account searching for user IDs: https://github.com/matrix-org/synapse/blob/ed630ea17c40d328cc0796e35d37287768c7140d/synapse/storage/data_stores/main/user_directory.py#L718-L773

(The user_id bit at the top only being for excluding the current user from search results and/or user whom the requester does not search a public room with, depending on the value of user_directory_search_all_users).

The query should be updated to search by the user ID.

Additionally, we don't do anything with respect to "preferably using a collation determined based upon the Accept-Language header provided in the request, if present", but as we're just searching user ID parts here, I'm not sure how that applies.

The tsquery bit specifies 'english'. Perhaps we should be modifying that based on Accept-Language headers.

Synapse v1.14.0.