Replaces 'do_unmute_topic' with 'do_set_user_topic_visibility_policy'
and associated minor changes.
This change is made to align with the plan to use a single function
'do_set_user_topic_visibility_policy' to manage
user_topic - visibility_policy changes and corresponding event
generation.
This commit is a step in the direction of having a common
function to handle visibility_policy changes and event
generation instead of separate functions for each
visibility policy.
In order to support different types of topic visibility policies,
this renames 'do_topic_mute' to 'do_set_user_topic_visibility_policy'
and refactors it to accept a parameter 'visibility_policy'.
The "add_topic_mute" and "remove_topic_mute" library functions
shouldn't be called directly from tests.
They should instead call "do_mute_topic" and "do_unmute_topic"
The reason being:
Library functions are meant to be internal interfaces
for just changing the database, and shouldn't generally be
called elsewhere.
This makes use of the new case insensitive UNIQUE index added in the
earlier commit. With that index present, we can now rely solely on the
database to correctly identify duplicates and throw integrity errors as
required.
