Resources tags

[Xpirix]

This is the proposed fix for #393

Changes summary

• Add tags feature for all resource types (Styles, Projects, Models, 3D models, Layer definition)
• New optional field tags in the upload and update form

Please find below a screen recording of the tags feature for the styles hub as an example:

style-tags.mov

[dimasciput]

@Xpirix could you add more tests please? Especially for this function qgis-app/models/templatetags/resources_tagcloud.py

[Xpirix]

@dimasciput Sure, I'll do that

[nyalldawson]

Thank you! This looks great!