Require a POST request to create identifiers

Implement a requirement for POST requests when creating identifiers, which helps mitigate the risk of web crawlers creating user accounts and accessing the bug tracker through anonymous accounts.

To achieve this, it replaces links to the create_identifier endpoint with <form> elements, as this is the only approach I am aware of to enable a POST request without relying on JavaScript.

Please be aware that this change has resulted in some CSS inconsistencies in font-size and line-height alignments that may require additional review.