Switch between working directories

[carlosms]

From the design doc:

As a user I want to choose the dataset that I will be working with so that I can switch between sets of repositories for different usages

From a previous conversation:

@carlosms

One thing to discuss. If you change the repositories dir, should the Superset DB also change?

i.e. if I start the sandbox with the repos in bblfsh org and I create a dashboard with charts that show certain metrics.
Then I change the repositories dir to work with src-d repos.
When I log in back to Superset, is it expected to have the same dashboard with updated information, or is it supposed to be a clean installation?

@mcuadros

The same dashboards, you should keep the postgresql

But in a later informal discussion it was mentioned that the other scenario might be preferred.
Needs input from @src-d/product @mcuadros @smola

[smacker]

keep in mind to clean redis no matter do we keep postgresql storage or not.

[ricardobaeta]

I believe the user could benefit from the option to choose from both, keeping the same dashboard with updated information as default.

[smola]

As discussed in our last Apps sync (https://github.com/src-d/minutes/pull/577), preserving some state between working directories (e.g. PostgreSQL database) and not other (e.g. git repositories) gets quite tricky, and it's quite hard for us to envision the major use cases for each behavior.

So the simplest solution would work for us initially: allow having a completely separate compose project, with no fancy magic regarding state management. We can figure out better for future releases.

[carlosms]

Code and docs already in master.