manifest.common.json: disallow running in incognito mode

[Mikaela]

I am not entirely sure this came to the correct spot as there are no examples that I could see and I think incognito could logically be a permission.

Requested in #704 (comment). Closes #704

[lidel]

Thank you @Mikaela! I tested it with Firefox Nightly and worked as expected.

There is one nuance to address here: we need to set minimal version of Firefox to be 67.
If we ship this with <67, then extension will fail to load with this error:

Error: unknownError: Could not install add-on at '/home/lidel/project/ipfs-companion/add-on': Error: Extension is invalid

Can you please update this PR and bump the minimal versions in below places to 67?

https://github.com/ipfs-shipyard/ipfs-companion/blob/8048f9f3a1bda5cf13987236f6d91d4136521ccf/add-on/manifest.firefox.json#L11

https://github.com/ipfs-shipyard/ipfs-companion/blob/8048f9f3a1bda5cf13987236f6d91d4136521ccf/.babelrc#L10

This way we will ensure update will get in effect only when user updates to Firefox 67.

ps. don't worry about failing Travis tests, they fail because WebExtension linter is not aware of updates in Firefox 67. We will update it before this PR is merged.

[Mikaela]

Done, however this may drop privacy.resistfingerprinting users to old version until Firefox ESR updates to 67 or higher thanks to Firefox bug 1394448: Cannot install Addon withprivacy.resistFingerprinting==true.

As a workaround I think I am switching back to the beta channel which I left only because it doesn't work with Firefox Sync resulting me either not having IPFS Companion at all or having two of them.

[lidel]

Thank you @Mikaela

You raised a valid concern: we don't want to force people to switch to different browser channel if they want to use Companion Beta. Companion Beta should always work with Stable Firefox.

I believe a safe way to ship this is to park this PR until Firefox 67 lands in Stable by the end of this quarter. Firefox 68 will become ESR in July soon after that:

Does this sound ok?

[Mikaela]

I think it sounds ok (but I am staying in the beta as I am curious enough and following enough issues).

I think you may be overestimating my skills/abilities/knowledge on Firefox/webextensions/development and spent a long time staring at documentation after you asked me to look into this in #704 (without which I wouldn't have done anything).

[lidel]

Firefox 68 landed to ESR channel, let's merge this! :)