Bugfix `@service` being nil in integrations views

What does this MR do?

This fixes an error when users view any instance or group-level integrations that have a @service variable in their view templates.

We renamed a variable @service to @integration in 5fd1c950 in IntegrationsActions (a concern used by instance and group-level integrations controllers). We need to continue to support that variable as it is referred to in shared views that instance and group-level integrations use, such as app/views/projects/services/mattermost_slash_commands/_help.html.haml.

This fix applies the same approach as in the original commit to continue to support this variable for project-level integrations. Removing the need for supporting @service will be addressed in #329759 (closed).

It also adds a basic framework for group and instance-level integration tests with an initial test of the Mattermost Slash Command integration (one of the integrations that broke). A follow-up issue will address the remaining holes in our test suite #331325 (closed).

Issue: #331262 (closed)

Before merging

We should apply the Pick into 13.12 label when this MR is ready to be merged to master !62054 (comment 578960963).

QA steps

1. Go to Admin -> Settings -> Integrations
2. Click on Mattermost slash commands
3. The integration should render fine. Previously this caused a 500 error: undefined method project_level? for nil:NilClass

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
• [-] I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• [-] I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Related to #331262 (closed)