The "subdomain" label is redundant, to the extent it's even
accurate -- this is really just the URL we want to display,
which may or may not involve a subdomain. Similarly "external".
The former `external_api_path_subdomain` was never a path -- it's a
host, followed by a path, which together form a scheme-relative URL.
I'm not quite convinced that value is actually the right thing in
2 of the 3 places we use it, but fixing that can start by giving an
accurate name to the thing we have.
This was part of the logic to handle EXTERNAL_API_PATH varying.
But also it was already no longer used -- it was only ever passed
into template contexts, as `external_api_uri`, and it'd been
overtaken there by `external_api_uri_subdomain`.
So, update our dev docs to reflect that, and eliminate the variable.
Update Email, Beanstalk, Hubot, JIRA, and Trello integrations
links.
The Hubot integrations section (/integrations#hubot-integrations)
was removed in an earlier redesign of /integrations. This commit
replaces the link with the hubot-scripts organization on
Github, which displays the comprehensive list of all integrations
available via Hubot.
Fixes#5875.