[Dashboard] Implement /select-account

[svenefftinge]

1. First of, ensure you have two accounts with distinct provider connections, e.g. one with GitHub and one with GitLab.
2. Log in with user A.
3. Go to Integerations, try to connect with second provider (which is connected to user B)

You end up on an empty /select-account page.

Designs

Modal	Modal (Spacing)

See also design specs.

[AlexTugarev]

@gtsiolis is this design spec still valid? #3072

[gtsiolis]

@AlexTugarev Oh, specs in #3072 are obsolete but I can add them within the next hour if needed.

To allow some head start, this page could use the following patterns:

1. A modal.
2. Two list entries, as seen in Existing Workspaces specs in [Dashboard] Workspace Start Page #3301, with all the needed accout information on each entry.
3. Two action buttons as seen in the existing page.

[AlexTugarev]

I can add them within the next hour if needed.

I think this would be very helpful.

[gtsiolis]

@AlexTugarev I've added the design specs for this in the description above but wanted to provide also an alterative design that would reuse existing patterns and components in case this could help us implement this faster.

Proposal A	Proposal B