Keywords cleanup

[zzgvh]

Test plan

1. Run the script as a standard Django managent command: manage.py merge_keywords
2. The keywords should be merged (as mentioned in the file @lauraroverts provided)

Issue description

Going forward the project keywords are to be used to gather the projects shown on an Akvo Site. This replaces the use of sponsor partners for this type of grouping.

As a step in the removal of sponsor partners, all projects associated with a sponsor partner was given a keyword constructed from the ID and the name of the partner organisation. This ensured that no projects were left out, but also has lead to a number of projects, that were already tagged, e.g. with a consortium keyword, to acquire duplicate keywords.

To fix this we need a data migration that merges duplicate keywords. There are also a number of keywords that aren't used any more. Those should be removed. Finally there are some special cases where a number of Walking for Water keywords should be merged. @lauraroverts has made a list of changes that need to be made (its extension is .txt to allow uploading, and it's semicolon separated).

[zzgvh]

I've seen there are a couple of keywords with an ID prepended still, after the merge. Those can be removed in the admin, after making sure all projects with such a keyword has at least one other keyword.