Merge "up-for-grabs" and "first-timers-only" labels into "Accepting Merge Requests"

Idea

• remove up-for-grabs label and treat Accepting Merge Requests issues with weight smaller than 5 as "suitable for people that never contributed to GitLab"
• remove first-timers-only label and treat Accepting Merge Requests issues with weight 1 as "suitable for people that never contributed to open source"

Reasoning

Please refer to #891 (comment 18912052)

Steps (order matters)

1. describe Accepting Merge Requests in https://about.gitlab.com/handbook/engineering/workflow/#labelling-issues
2. update https://gitlab.com/gitlab-org/gitlab-ce/blob/master/PROCESS.md#workflow-labels
3. communicate within the company
4. change description of Accepting Merge Requests label in gitlab-ce
5. go through all Accepting Merge Requests issues and check if they meet the criteria
6. go through all up-for-grabs issues, check if they meet the criteria, assign weight and change label to Accepting Merge Requests
7. go through all first-timers-only issues, check if they meet the criteria, assign weight and change label to Accepting Merge Requests
8. describe how to find issues in CONTRIBUTING.md
9. update https://about.gitlab.com/2015/12/16/improving-open-development-for-everyone/
10. update https://about.gitlab.com/2016/12/07/heres-how-new-programmers-can-learn-by-contributing-to-gitlab/
11. update https://github.com/up-for-grabs/up-for-grabs.net/blob/gh-pages/_data/projects/gitlab.yml
12. remove up-for-grabs label
13. remove first-timers-only label