Add Metafields to all the entities like in Categories

[deshetti]

Adding metafields to all the entities would make Dega extensible in a major way without adding specific fields/functionalities based on individual spaces.

Also, the current JSON editor that we have for adding metafields should be changed to something more user-friendly. Need to do some research on the same.

[deshetti]

Implemented as part of this issue: #133