Closes #7330

Changes proposed in this pull request:

• Default behaviour is same
• Added FreshRSS_Context::userConf()->html5_enable_notif so that, it determines weather user wants notification. (will not show any even it has permission)
• Added button such that checking it makes it request permission too

• clear commit messages
• code manually tested

Issue:

• for new users the default FreshRSS_Context::userConf()->html5_enable_notif starts with true , so default behaviour will be same
• but old users , this field may be missing in their config,(they will lose notification, need to manually enable it) which may confuse them. is there way to migrate the config file too.

Edit:

• I inverted variable names html5_enable_notif is now not_html5_enable_notif , so default false on file is true.