Small visual tweaks to slack/webhook UI

[snipe]

These are just some small tweaks - possibly not the last - for the new Livewire webhook form. @Godmartinz - as far as I know, this doesn't break anything new. (See the video in Slack) It just cleans up some of the HTML. We were missing (and I may not have hit them all) some aria label/accessibility stuff, etc. I just wanted to put this out there so you could work from this and not have a giant mess of conflicts to deal with later today.

[what-the-diff]

• Added a new option to the dropdown menu for Slack and Discord
• Changed some of the text in general.php from "Slack" to "$webhook_selected" so that it can be used with both apps
• Updated slack-settings-form blade file, added an if statement around webhook channel input field (line 57) because this is not needed when using discord integration
• Removed unnecessary code on line 61 - 64 as well as lines 67 - 70 since they are no longer needed due to changes made above