webmail: implement registering and handling "mailto:" links

to start composing a message.

the help popup now has a button to register the "mailto:" links with the mox
webmail (typically only works over https, not all browsers support it).

the mailto links are specified in 6068. we support the to/cc/bcc/subject/body
parameters. other parameters should be seen as custom headers, but we don't
support messages with custom headers at all at the moment, so we ignore them.

we now also turn text of the form "mailto:user@host" into a clickable link
(will not be too common). we could be recognizing any "x@x.x" as email address
and make them clickable in the future.

thanks to Hans-Jörg for explaining this functionality.

rfc/index.txt

@@ -10,7 +10,7 @@ Each tab-separated row has:
 - RFC title
 
 If the support status column value starts with a minus, it isn't included on
-the protocol page on the website. Valid words for implementaiton status:
+the protocol page on the website. Valid words for implementation status:
 - Yes, support is deemed complete
 - Partial, support is partial, more work can be done
 - Roadmap, no support, but it is planned
@@ -389,6 +389,7 @@ See implementation guide, https://jmap.io/server.html
 3339	-?	-	Date and Time on the Internet: Timestamps
 3986	-?	-	Uniform Resource Identifier (URI): Generic Syntax
 5617	-?	-	(Historic) DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP)
+6068	-Yes	-	The 'mailto' URI Scheme
 6186	-?	-	(not used in practice) Use of SRV Records for Locating Email Submission/Access Services
 7817	-?	-	Updated Transport Layer Security (TLS) Server Identity Check Procedure for Email-Related Protocols