Leaderboard: show column label instead of column title

@ mention of reviewers`

@Didayolo`

A brief description of the purpose of the changes contained in this PR.

As mentions in #375 (closed) : In leaderboard, column key/title was used instead of label.

Now with the new changes, the following will happen:

1. if a column has label, this will be shown in the leaderboard. Example below:

2. if a column has no label, its title/key is used in the leaderboard. Example below

3. For old competitions, where we don't have label in db, title/key will be shown

Changes made:

1. Column model changed -> added new field label
2. Code adjusted to show label when available otherwise show title

Checklist

• Code review by me
• Hand tested by me
• I'm proud of my work
• Code review by reviewer
• Hand tested by reviewer
• CircleCi tests are passing
• Ready to merge