[#1780] Add Welsh translation for DB form

[mdeuk]

Relevant issue(s)

Fixes #1780
Fixes https://github.com/mysociety/whatdotheyknow-private/issues/262
#1775

What does this do?

1. Reconciles changes for /lib/views/help/contact.html.erb from Signpost data breach form #1775 into Welsh translation.
2. Adds partial translation for /lib/views/help/report_a_data_breach.html.erb

Why was this needed?

Data breach form was not translated to Welsh.

Implementation notes

The breach form, itself, is only a partial translation, as:

• the heading, at level 1, is not translated
• the further information text, and everything following, is not translated
• related file report_a_data_breach_thank_you.html.erb is not translated

Screenshots

N/A

Notes to reviewer

I had a bit of trouble with Git commits - hopefully all makes sense.

We will need to follow-up on the remaining parts, but this can be done later on.