Change the build process for v2.6

[ameshkov]

Build configurations

We should provide 3 different build configurations (just like we do now).

1. Dev

chrome
edge
opera
firefox-amo
firefox-standalone
firefox-legacy
safari

2. Beta

chrome-beta.zip
edge-beta
opera-beta
firefox-amo-beta
firefox-legacy-beta
firefox-standalone-beta
safari-beta

3. Release

chrome-release.zip
edge-release
opera-release
firefox-amo-beta-release
firefox-amo-release
firefox-legacy-release
firefox-standalone-release
safari-release

Self-hosted extensions

1. Do not use chrome.adtidy.org for hosting extensions anymore.

2. Use Github Pages branch to serve updates manifest files for self-hosted extensions.

   Which extensions are self-hosted:
   firefox-legacy-beta
firefox-standalone-beta
safari-beta

3. Build output should contain update.rdf, update.xml and such.