[Form editor] Allow user to add Number fields

[gino-m]

This involves adding a new type to the field type dropdown "Number".

Icon:
https://fonts.google.com/icons?icon.query=number

Field type enum value in remote db:
number

UI will be identical to that for adding text fields.

[gino-m]

@sergeydolgov1 is this a priority for pilot or RC?

[gino-m]

Imo this is needed for feature completeness during the pilot - the number of form field types would also look very sparse without it. Moving to pilot backlog.

[gino-m]

Fixed by #660