Important steps for successful Piwik 3.0.0-b1 release

Prior to release 3.0.0-b1 we must:

• Before merging Piwik 3.0 into master we need to: make all submodule plugins compatible with 3.0.
• developer.piwik.org: once Piwik 3 is into master, we need to increase the version number so our developer documentation display the Version selector for this new major release (Support switching between Piwik versions developer-documentation#138)
• search in codebase for 3.0.0- and update the code - likely these occurences of 3.0.0- should be replaced with 4.0.0- ?
• api.piwik.org: make sure that api.piwik.org service returns correct 2.X beta, on
  Latest 2.X beta channel, should NOT return 3.0.0-b1. URL: http://api.piwik.org/1.0/getLatestVersion/?release_channel=latest_2x_beta
• api.piwik.org: make sure standard channel returns latest 3.0.0-b1 ONLY IF php 5.5 or later http://api.piwik.org/1.0/getLatestVersion/?release_channel=latest_beta
• Create a new guide "Migrate Plugin from Piwik 2.X to Piwik 3" (done in Support switching between Piwik versions developer-documentation#139)
• Move this checklist to a wiki page so we can re-use the knowledge when we'll release next major release Piwik 4 as we'll have to do all these steps again
• update piwik package script, we need to edit build script https://github.com/piwik/piwik-package/blob/master/scripts/build-package.sh ...

SEE THIS https://github.com/piwik/piwik/wiki/New-major-releases

(from #8688 )