Repository links in the student view appear to be broken about quickfeed

[0xf8f8ff]

The reason for this:

Backend makes Repositries.URLs (map[string]string) with urls[repoType.String()] = repo.GetHTMLURL().
Frontend receives this in the following format:

repos: 

["INFO"]: <URL>,
["ASSIGNMENTS"]: <URL>,
["USER"]: <URL>

However, it accesses the map with, for example, repo[Repository_Type.INFO]} and so on, which is repos[1] and a value for this key does not exist in the map.

[meling]

Seems like the appropriate fix for this would be something like:

message Repositories {
    map<Repository.Type, string> URLs = 1;
}

I guess this issue could be fixed in #942 as well, since they are related.

[0xf8f8ff]

map<Repository.Type, string> URLs = 1;

Repository.Type won't compile because it is Enum, but, for example, uint32 works just fine.