Currently we can’t add a case-sensitive watched word if another one
exists with a different case. For example, the existing watched word
`Meta` has been created and is case-sensitive. Now an admin tries to add
`metA` while marking it as case-sensitive too, this won’t work and the
word won’t be added.
This patch changes this behavior by allowing to add same words that have
different cases, so the example above will now work as expected.
We still check for uniqueness but case-sensitivy is now taken
into account. It means that if the watched word `meta` already exists
and is not case-sensitive then it will not be possible to add `Meta`
(case-sensitive or not) as `meta` already matches every possible
variations of this word.
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