Move Web Terminal for Web IDE to Core
### Overview
As part of an [effort to open source](https://gitlab.com/groups/gitlab-com/packaging-and-pricing/-/epics/1) some currently paid features and move them to our free tier the Web Terminal for Web IDE has been identified as a feature worth moving down.
### Proposal
The Web Terminal for Web IDE should be moved to core. This requires the removal of any licensing checks for the feature which is currently in the Ultimate tier.
### Documentation
Documentation for this feature should be updated to reflect that this is no longer in a paid tier. However, the feature is not available to all users as it still requires specific runners for use of the feature.
### Testing Activity
* Check web terminal works on Core (@lulalala)
* We can reuse the existing specs (@fjsanpedro)
### Availability and Testing
End-to-end test for Web Terminal is required. As of now, we only have `web_terminal_spec.rb` (EE only) that is permanently quarantined due to https://gitlab.com/gitlab-org/gitlab/issues/13833. We should investigate if refactoring of the old test is an option or a new test needs to be created.
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