Resolves #38: Revalidate target URLs when handling pending mentions

• Should be unnecessary generally (they are also validated at discovery time when parsed from HTML) but important if validation checks are updated.

Resolves #41: Find correct endpoint when multiple links in HTTP header.

Added settings.WEBMENTIONS_INCOMING_TARGET_MODEL_REQUIRED [bool | default=False]. If True, incoming mentions are only accepted if their target resolves to a MentionableMixin instance.

Added settings.WEBMENTIONS_ALLOW_SELF_MENTIONS [bool | default=True].

• If False, outgoing links that target your own domain (as specified by settings.DOMAIN_NAME) will be ignored - you will only submit mentions to other domains.
• If True, outgoing links that use a relative path (e.g. href="/article/1/") are now supported.

Fix: WebmentionHeadMiddleware no longer overwrites existing links in HTTP header when adding webmention endpoint.

Fix: Webmention ’notes’ no longer persists across instances.