Import preview page

[pavish]

Related to #295
Fixes #296
Fixes #548
Fixes #524

This PR has:

• Refactoring of stores and pages to be schema centric, instead of db
• Preview page for file import
• Import status indicator
• Switches open tables when moving to other schemas/db

Note:

• This PR has all the UI changes needed for the preview page, but there are issues/changes needed with some backend apis.
• This leads to some features not working due to 500s from api calls.
  • Changing column names/types is not working for both:
    • Tried with tables PATCH, as mentioned on import wiki
    • Tried with columns API PATCH (Which is currently used)
  • Updating import_verified is not allowed (Not done at the moment)
• Does not focus much on styling. It will be done incrementally with future PRs.

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the master branch of the repository
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

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[kgodey]

@pavish This looks good to me, but all columns are still showing up as VARCHAR instead of the inferred type. It doesn't look like you're looking at the type_suggestions endpoint for the table to get the appropriate types for the preview. I don't know if we actually documented that endpoint anywhere, sorry about that.

Do you want to fix that in this PR or a future PR?

[pavish]

@kgodey I was not aware of the type_suggestions endpoint. I've marked this PR as related to #295, and not as fixes.
We could create a separate issue for the type_suggestions and I'll pick that up.

[kgodey]

@pavish Okay, thanks. I'll create a separate issue for that and merge this PR.

[kgodey]

I made #568 for the type suggestions work.