#26 move `options_ui` into extension page

Web extensions can have a dedicated tab for settings / preferences in the Extensions page ( about:addons ). While the settings page was already set up for `options_ui`, it was set to open in a dedicated tab. In my humble opinion, users would expect the settings for the extension to be in the Extensions UI rather than a dedicated tab. This change-set moves the settings page into the Extensions page. Though this also invalidates the need for a dedicated Settings button in the popup UI, I simply changed the reference the button points to. We could possibly remove it since the settings is now in a place that is accessible ( and expected ) by the users. I wanted to merge the rules page into this page and have all the settings in one place, but since that idea was [not accepted favorably](https://notabug.org/nipos/cloud-firewall/pulls/24#issuecomment-23953), there is not anything more left to add to this page now. If there are not any major issues, it would be great if this change went into the next release too.