Slack notifications: Rename misleading #general

Description

Following up on issue gitlab-foss#20707 (closed) , the default settings for the slack integration page are misleading. According to the slack service documentation, the default channel is "#general". This has two issues:

a hashtag in front of a word may be a channel, but can also simply denote a topic (tagging)
when entering a channel, the documentation informs you, that it has to be done without a hashtag in front - but the placeholder does have one
when using with the integrated mattermost, adding an incoming webhook requires you to set a channel for this webhook. Setting a non-existing channel here does not result in an error message, but the webhook to simply not work.

Also, clicking the "Test Integration" button (secondary option on the page) does not save any data and discards all changes without notice.

Set the placeholder to "channel..." instead of "#general"
Rename the secondary action button from "Test Integration" to "Save and Send Test Notification" and make the button also save current changes
Properly handle error messages during test integration (http status is 500, if the channel does not exist)