Prevent creation of duplicate tags

[allyoucanmap]

Description

Currently it is possible to create multiple tags with the same name. Both backend and frontend should implement the logic to avoid this:

• backend, check if the name for a tag already exist and thrown an error (similar logic for resource name)
• frontend, manage the error in the UI

How to reproduce

• login as admin
• access to the tags manager in the manager menu
• create a tag
• create a new tag with the same name

Expected Result

An error in the UI stating the name already exist

Current Result

A duplicated tag is created

• Not browser related

Browser info

(use this site: https://www.whatsmybrowser.org/ for non expert users)

Browser Affected	Version
Internet Explorer
Edge
Chrome
Firefox
Safari

Other useful information

[axl8713]

PR for the backend here: geosolutions-it/geostore#416

[offtherailz]

@ElenaGallo, could you please test this on DEV ? Thank you