[GEOS-11370] Refactor inline JavaScript in the TestWfsPost Page

[sikeoka]

This PR moves the inline JavaScript in the TestWfsPost HTML page into an external file. This PR is related to Content-Security-Policy work for GeoServer 2.26.0 and should NOT be backported.

The new webresources path is intended to be reused which is why it was put in gs-main. The additional HTML escaping isn't really necessary but is a good practice.

Checklist

• I have read the contribution guidelines.
• I have sent a Contribution Licence Agreement (not required for small changes, e.g., fixing typos in documentation).
• First PR targets the main branch (backports managed later; ignore for branch specific issues).
• All the build checks are green (see automated QA checks).

For core and extension modules:

• New unit tests have been added covering the changes.
• Documentation has been updated (if change is visible to end users).
• The REST API docs have been updated (when changing configuration objects or the REST controllers).
• There is an issue in the GeoServer Jira (except for changes that do not affect administrators or end users in any way).
• Commit message(s) must be in the form [GEOS-XYZWV] Title of the Jira ticket.
• Bug fixes and small new features are presented as a single commit.
• Each commit has a single objective (if there are multiple commits, each has a separate JIRA ticket describing its goal).

[petersmythe]

Merging conflict manually in GitHub IDE

[petersmythe]

LGTM, I built & tested it locally