[webapp] Add Max/Path Policy Feature

[mwfarb]

Currently when requesting paths using webapp, the default set of paths is 5 and for a User AS on SCIONLab there is no path policy configured for the endhost user to easily test. However, there is some ability to control paths at the application level if the command-line app has implemented an interactive path selection feature.

This enhancement would add a control panel to allow users to experiment with more path control beyond these features. In general the additions would be:

• Update Apps page Path Selection panel to alter the max. paths beyond the default 5.
• A new nav bar page named Control to give users the ability to easily edit path_policy.yml.
• Control page would have drop down selection of Path Policies: none (default), 1GB BW, 10GB BW, Video Conference, File Transfer, etc...; each of which could have pre-selected values.
• In addition, users could custom edit and save/name/recall path_policy.yml.
• Selected policies in use, or in edit mode, would have each property displayed with the assigned values for review.
• Each property of path_policy.yml on display should have an information icon to click for a tool-tip explaining the definition of the property as well as the implications for changing it and common ranges.
• Predefined policies and user edited policies could be stored in the webapp database and generated into .yml format JIT such that webapp could manage any format changes to path_policy.yml in the future for backward-compatibility.