Refactors common code into own components

[Namoshek]

This PR refactors some of the existing component templates into sub-components. This is especially true for the Dash and Dashboard components.

Besides the refactoring, I also made following changes:

• added an actual user menu which opens when the username in the top right corner is clicked
• made the messages in the top bar render from the storage and removed the margin on the left side of each message where normally the users avatar would be
• put the alert box on the dashboard into a <div class="col-xs-12"></div> for nicer looks
• removed some unused variables (e.g. faker variables in Dash.vue)
• replaced multiple props of Sidebar.vue with one single user prop (as the same is used for the DashHeader.vue now)
• replaced the simple notifications menu with a more complex but nice view that has two tabs, one for old notifications and one for new ones. The new menu also has a button to mark all new notifications as read, etc. I developed it for a working application and use it in production but with some different capabilities (e.g. infinite-scrolling for old notifications).
  

Hopefully, I did not forget to mention anything.

[misterGF]

Thanks for the PR. Please give me a few days to test it out.

[misterGF]

This looks great @Namoshek! One thing that I ask if that you get rid of the scss css in some of the new pages.

Most of our windows users have been having issues with the node-saas package and I decided to remove it as a devdependency in the base project.

My latest commit removes it from package.json and the Dash.vue page.
This is a good conversion tool- https://www.sassmeister.com/.

Please also push to dev branch. Thanks!

[Namoshek]

Thanks for having a look at it @misterGF! I added the changes you requested and rebased on branch dev.