Condense project selector stores/instances

The following discussion from !64560 (merged) should be addressed:

• @aturinske started a discussion: (+1 comment)

  This required a lot of methods and that was annoying. Especially since I copied all of them the instance-level security dashboard component and they all seem to work. Maybe (as a follow-up) I should create a wrapper around project-selector that encapsulates all these methods for these two instances? Counter-point: There are only two instances, so maybe do that if there is a third instance? There are actually two other instances of the project-selector (instance-level operations and environment dashboards), but both of those use a Vuex store. Oh, there is a project_selctor Vuex store, should I be using that instead of these copy/pasted methods?

  I will...look into this.

Implementation

I talked to @ekigbo about this and we decided on a two pronged approach

• 1. Work on trying to merge the Vuex stores together so that the one in vue_shared is left. This can start by merging the actions; a lot of the actions are the exact same so import the actions from the vue_shared one into the other and delete the repeated actions. Then simplify any extra actions that do anything different and use that actions file as an override file. Hopefully by then we will be able to see if these Vuex stores can be condensed into one or if they need to be separate Delete the security dashboard project selector Vuex store as it is only used in https://gitlab.com/gitlab-org/gitlab/blob/master/ee/spec/frontend/security_dashboard/store/modules/vulnerable_projects/mutations_spec.js, which seems like a mistake.
• 2. Replace all the reused logic in instance-level security dashboard project manager with the newly created instance_project_selector component.
• As a side note, we should also create storybook stories for the project_selector
• Friendly reminder to make sure to ask for a UX feedback after submitting the MR.

Additional context

The current implementation has a help text. We want to keep that text.

current project selector	proposed project selector