Piwik will chmod files created in tmp/* 750 instead of 755

[mattab]

For increasing security, Piwik should chmod files in tmp/ folder with 0750 permission (instead of current 755).

• Update chmod commands
• Update the messages shown on screen that mention "chmod"

follows up #1833, #4046

[mattab]

In f1f4a7a: Fixes #5034 chmod files created in tmp/* as 0750 instead of 0755
The code should fallback to 0755 if the webserver user still cannot write to the directory.

[mattab]

In 6c6e5e9: Refs #5034 Only restrict chmod 0750 to folders in tmp/*
It causes otherwise issues with folders in plugins/* which serve assets such as HTML files (for AngularJS modules)