Using lazy_gettext in boot-time translations

[mesemus]

Description

This pull request fixes premature evaluation of i18n strings inside static fields. gettext call, that causes
the evaluation to be performed at boot time, was replaced with gettext_lazy that is performed at
request time. This fixes sort options not being translated when language is changed.

Checklist

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• I've marked translation strings.
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[mesemus]

Please backport this change to RDM 12

[Samk13]

LGTM! Thanks for the great addition!

I noticed that some packages register translations as:

invenio_i18n.translations =
    messages = invenio_i18n

while others use:

invenio_i18n.translations =
    invenio_i18n = invenio_i18n

see: here
For clarity and consistency, we should standardize on invenio_pkg_name = invenio_pkg_name across packages, although it doesn't make a big difference here. Using the package name as the namespace avoids ambiguity and prevents potential conflicts.