Two hook implementations alter the site unconditionally

Problem/Motivation

Two hook implementations in myrest.module change the behaviour of the whole site rather than of this module's endpoints.

• myrest_mail_alter() replaces $message['body'][0] of every contact message with key page_mail. A site that uses the core contact form for anything — a support address, an editorial workflow — silently loses the body of its mail and gets this module's sentence instead. Nothing in the interface says so.
• myrest_toolbar_alter() attaches a library to $items['admin_toolbar_tools'] without checking that the key exists. Without the admin_toolbar_tools module it creates a toolbar item consisting of nothing but an attachment.

Proposed resolution

• Scope the mail alter to messages this module sent: mark them when the submission resource sends them and act only on that mark. A site whose own contact form sends mail is untouched.
• Decide whether the replacement text is wanted at all, or whether it belongs to the resource that composes the message; the current text describes a form submission, which is what the resource does.
• Guard the toolbar alter with a check for the item, and for the module that provides it.
• Cover both: a contact message sent outside this module keeps its body, and the toolbar alter is a no-op without the toolbar module.

Remaining tasks

Everything. Found while auditing what the module changes outside its own endpoints.