linkifiers: Extract shared parameter in `zulip.yaml`.
linkifiers: Extract shared parameter in `zulip.yaml`.

In zulip.yaml, `pattern` and `url_format_string` are extracted
as a shared parameters.

This is done as a preparatory commit for `Add settings UI to edit
linkifiers`.

Related issue: #10830.

kou/zulip,zulip/zulip,hackerkid/zulip,andersk/zulip,zulip/zulip,rht/zulip,eeshangarg/zulip,andersk/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,rht/zulip,zulip/zulip,andersk/zulip,hackerkid/zulip,hackerkid/zulip,kou/zulip,andersk/zulip,rht/zulip,zulip/zulip,punchagan/zulip,zulip/zulip,kou/zulip,hackerkid/zulip,kou/zulip,punchagan/zulip,andersk/zulip,andersk/zulip,hackerkid/zulip,hackerkid/zulip,eeshangarg/zulip,zulip/zulip,rht/zulip,punchagan/zulip,punchagan/zulip,andersk/zulip,punchagan/zulip,kou/zulip,hackerkid/zulip,eeshangarg/zulip,rht/zulip,eeshangarg/zulip,punchagan/zulip,eeshangarg/zulip,zulip/zulip,punchagan/zulip,kou/zulip,eeshangarg/zulip,rht/zulip