Don't require tags, but require skills when setting up a challenge

[jmgasper]

REF: https://topcoder.atlassian.net/browse/TSJR-248

When setting up a challenge, we are currently requiring a user enter a tag, but not skills.

We are going to change this to:

1. No tags are required
2. At least one skill is required

[suppermancool]

@jmgasper done in work-manager-issue-1576.patch (this patch against commit b10ce0327f05de4b060f3570816fc8fdb6afcf2f in develop branch)

I implemented this in frontend ui but the PUT /v5/challenges/{...} api required tags field: