Fix issue #308: ability for a user to select if he/she wants to see random tips

[DovydasGrb1]

According to Issue 308, there was no functionality to disable random tips (which had 20% chance of appearing), therefore, I have added a checkbox in the popup which allows the user to disable random tips.

[rugk]

Hi @DovydasGrb1,
first of all, thanks for your first contribution to this project! 🎉 👍 🏅
I hope you'll like this project and enjoy hacking on it… 😃

Note you can (automatically) let issues close when a PR is merged by adding some "magic" text to your PR body. (Manually linked it now.)

[rugk]

Please don't add this checkbox to the popup window though. If it is a useful setting for you I'd accept it in the actual settings of the extension, but not in the popup. The reason is the popup should be as slim/minimalistic as possible, so a checkbox, which is always shown, is certainly not good. Especially, if, as you correctly explain, the tip is only shown in 20% of the cases (if there is actually any one), but now you have a checkbox "in the way" for 100% of the time. 🙃

Additionally, please don't manually call the respective APIs for saving the options, we use a library here for that, but you'll see how that is done in the options HTML etc.. More information/documentation here: https://github.com/TinyWebEx/AutomaticSettings/