Create a nicer UI for importing

[mauritsvanrees]

This is related to pull request #16, which already updates the Import tab in the ZMI.

Perceived problem: the Import tab is too complicated.

Initial idea:

• Move the tarball import to a separate tab. Any checkboxes or other options on the current Import tab have no influence at all on importing a tarball. Tab name: Tarball or Archive.
• The rest of the Import tab can stay. The changes I did to the UI in pull request Apply profile dependencies only once. #16 can probably be reverted for this tab. Maybe remove 'Import all steps' and instead have a 'select/deselect steps' button/checkbox, although that might mean adding some javascript, which is currently totally not in use. Rename the 'Include dependencies?' checkbox to 'Include dependencies of selected steps?', because that is what this checkbox does. It has nothing to do with dependency profiles, if you ever thought that. In other words, the Import tab is for when you want to have full control over which steps are imported.
• Create a new tab, say titled Addons. Start with a list of profiles that have upgrade steps that can be run. Then have a list of profiles that can be applied. Add the new UI options from pull request Apply profile dependencies only once. #16 here, giving control over how dependency profiles are handled. Maybe we can actually show the list of dependency profiles and give their current status: installed / not installed / needs upgrade. Do not give the option to select individual import steps: this page is for importing a full profile.

Given the new Addons tab, it probably makes sense for the Import tab to not apply or upgrade any dependency profiles. We could warn when we see this is a problem, and point to the Addons tab.

I am willing to work on this.

[mauritsvanrees]

Ah, I knew I had noted some ideas somewhere and here they are. :-) I have meanwhile done part of the above in pull request #23.
This issue has partially different ideas, which may or may not be better, but let's close it in favour of actual code in that pull request.