Allow release form to receive tag from params

What does this MR do and why?

When users click on "Create release" on the tag page, we want to direct them to the "new release" form with tag_name preselected.

This MR:

• passes tag_name from URL parameter to the form
• explicitly sets "existing_release" if form is an edit form (previously we used tag_name for this)
• hides the "Create from" from field if we set tag_name using URL parameter

Screenshots or screen recordings

You can see how it will be used in the Redirect editing release notes to proper forms (!88832 - merged)

https://www.loom.com/share/0ab892fa047f459e8dde784947417c73

How to set up and validate locally

Just go to new release page, and manually add ?tag_name=existingtag to the URL.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #362339 (closed)