login button in TAFXplorer

[lucalamoni]

Add login button on the right side of the page. Consider reusing the popup of previous version of TAFXplorer. Potential further improvements

• add button to navbar
• change icon after successful login

[colinpmillar]

Login implemented as modal window at page load.

[colinpmillar]

Additional improvements

• [x] show logged in user name in navbar
• [ ] allow logout, possibly implemented as a page reload.