Make slack url translatable in the warning for posts held for moderation after 96 hours

[felipeelia]

When a post is held for moderation we currently inform users that it will be reviewed. After 96 hours that message changes to another version, with a link to #forums slack channel as seen here.

Although this message is actually never showed in the internacional forum (posts are never hold that time long), in more silent local forums it can happen. So making the url translatable could point users to local slack instances. That way we could reduce chances of users going to the international slack when there is a more appropriate place to ask for help.

p.s.: In Brazilian forums we ended up translating that just assigning the placeholder to another attribute, as can be seen ​here, but IMHO making the url translatable is the right thing to do here.

[tobifjellner]

For the Swedish forum I'd love to be able to put a lower number than 96. With a stray entry in moderation once every 1-2 months, it would be good to have the user himself reminding us about posts pending moderation.

[SergeyBiryukov]

In 7129:

Support Theme: Make Slack URL and moderation timeframe in pending posts notice translatable.

Props felipeelia, tobifjellner.
Fixes #3583.