Ensure blessed Web UI is pinned

[lidel]

Context

We switched to loading Web UI from IPFS in #680.

Blessed CID is hardcoded in extension sources:

https://github.com/ipfs-shipyard/ipfs-companion/blob/1b81a4fac937fff36d8b88f2e48eb4ede6cfb58a/add-on/src/lib/state.js#L27

Problem

@Mikaela noted interesting problem:

Should the webui get automatically pinned or something? I have gc enabled on go-ipfs
I am wondering what happens if I get disconnected for two hours or so, will I lose the webui?

Solution

ipfs-companion should:

• keep current webuiCid pinned
  • check could happen every time remote API is connected to for the first time in browsing session
• when webuiCid changes, unpin old version and pin new one
  • relevant API: browser.runtime.onInstalled with OnInstalledReason

[lidel]

#735 introduces small optimization that preloads only the root of webui to speed up the first time it is opened.

Pinning is tricky, because don't want user to fetch entire webui right after extension is installed or when Web UI's Status page shows bandwidth chart with high load (here is why: ipfs/ipfs-webui#854, ipfs/ipfs-webui#848)

Ideas to explore:

• IIRC files in MFS are implicitly pinned (won't be GC'd) but do not need to be recursively present in local repo. This means we could add Web UI root to MFS and keep it from being GC'd that way.
  • Where to keep it? {MFS}/.cache/ipfs-webui ?