Show "manage sieve" button on account start page

[cweiske]

Prerequisites

• Tried the most recent nightly build
• Checked if your issue is already reported.

Is your feature request related to a problem?

I could not find the button/menu item that opens the sieve management window, just as #435.
I expected it on the account overview screen, because that one has a "manage message filters" button:

Describe the solution you'd like

Show the "Sieve Message Filters" button on the account overview screen.

[thsmi]

The webextension api does not provide any entry points to access this page.

But the new spaces sidebar has an api which can be customized by addons which looks like this:

Now the sad news, the spaces bar implementation is incompatible with tabs thus currently useless. There is a bug which is more or less actively worked on, but I is is unlikely what this api will be fixed before the next major Thunderbird release.

[thsmi]

duplicated by #807