fixed autocomplete for new topics is case-sensitive

[katisari]

#12725

Testing Plan:
Previously: When creating the same topic that already exists, the compose drop down menu will show two options:
"TOPIC", "topic"
when user tries to create a topic called "TOPIC" when "topic" name already exists in the database.

With the new changes to fix the issue, I tested by:

1. Created a new topic "topic" in my local repository in stream "test".
2. Tried to create new topic "TOPIC" in my local repository in stream "test".
3. Now, when user types "TOPIC", he/she only sees one option because I made the feature case-insensitive.

Solution Details:
My solution to the issue was comparing strings of the user input topic box with the topics in topic list by converting them to all to lowercase versions. Javascript's indexOf() is case sensitive, so I manually create a lambda function to manually traverse through the topic list to make the composebox typeahead case-insensitive.

This is an important feature to have because if there are two options for the same topic name, it creates confusion for the user. The user might think they he/she is creating a new topic, but in reality, his/her message will be added on to an existing topic if the topic names are the same (since the back-end is case-insensitive).

[katisari]

Feel free to leave a comment of the changes.

[vaibhavrajsingh2001]

@katisari try having a single commit that includes all changes.

[zulipbot]

Heads up @katisari, we just merged some commits that conflict with the changes your made in this pull request! You can review this repository's recent commits to see where the conflicts occur. Please rebase your feature branch against the upstream/master branch and resolve your pull request's merge conflicts accordingly.