linkifiers: Support URL percent-encoded bytes.

Supporting URL percent-encoded bytes is possible using `%%20`, but this
is not necessarily very understandable to end-users, even those that
understand percent encoding.

Allow `%20` in linkifier URL format strings, and transform them into
`%%20` in the pattern just before they are applied in markdown
translation.  Care must be taken here, such that already-escaped `%`s
are not escaped an extra time.

We do this before rendering, and not before storage, as
a simplification; the JS-side linkifier at present only understands
`%(foo)s` and thus needs no changes, and to avoid an un-escaping pass
before showing in the admin UI.

zerver/models.py

@@ -1138,6 +1138,9 @@ def filter_format_validator(value: str) -> None:
                   \( [a-zA-Z0-9_-]+ \) s  #   Interpolation group
                 |                         #     OR
                   %                       #   %%, which is an escaped %
+                |                         #     OR
+                  [0-9a-fA-F][0-9a-fA-F]  #   URL percent-encoded bytes, which we
+                                          #   special-case in markdown translation
                 )
             )+                            # Those happen one or more times
             $