unify storage of tag suggestions

[judell]

Problem you are trying to address with this feature

Because tag suggestions are remembered in local storage, they are machine- and browser-dependent. So suggestions differ from browser to browser. And if cookies and site data are blocked in a browser, they don't work at all.

Further, they are H-mode-dependent (the extension's tag memory lives in a different part of local storage from via's tag memory), which further fragments the experience.

A common experience of my-recently-used tags across AP/extension/via would be wonderful.

Your solution

Remember recently-used tags in the backend, provide a consistent experience across all client browsers and modes plus activity pages.