Mark strings for translation in `ee/app/views/projects/settings/slacks/edit.html.haml`

The following strings in ee/app/views/projects/settings/slacks/edit.html.haml need to be marked for translation:

• Edit project alias
• You can use this alias in your Slack commands
• Enter your alias
• Save changes

GitLab i18n guidelines for HAML files: https://docs.gitlab.com/ee/development/i18n/externalization.html#haml-files

Make sure to run bin/rake gettext:regenerate locally to generate an updated gitlab.pot file.