Admins write an HTML and a plain-text signature with placeholders on
Organisation > Signatures, per scope. Users fill in the details it asks
for once, under Settings > Account. Without a lock it is the default for
addresses without a signature of their own; locked or hidden, it
replaces every signature.
- `lib/org-signature.ts` fills the templates: values escaped, lines whose
placeholders are all empty dropped
- the composer applies it right before the plugin signature, so new mail,
replies, forwards, re-opened drafts, plain text and identity swaps all
take it the way they take an identity signature; quick reply too
- a send finaliser puts a locked signature back once when it was deleted
- the identity editor shows the filled-in signature read-only with "Set
by your organisation" when locked, and no editor when hidden
- `signatureDetails` is a synced setting
Linus Rath