ADMIN: Assign to tags and remove from tags (GSoC)

[meetulr]

Is your feature request related to a problem? Please describe.

Implements the UI and functionalities to assign people to tags and remove from tags.

Describe the solution you'd like

Add a modal on the manage tag screen that would list all the tags, from where we can select the tags and assign/remove them.

Additional Context

This is a companion issue of the one in the api: PalisadoesFoundation/talawa-api#2614

[meetulr]

Please assign it to me.