The support for injecting the thesis assignment PDF via the
`assignment=path` option of `\thesissetup` was added.

One of the larger design decisions in this commit is the fact
that each locale string stored within a `\macro` macro needs to
have a `\macroEn` counterpart defined. Such a solution was much
easier to implement and document than special-casing and
documenting each string, whose English version was needed across
locales. The implementation comprises `base.def` files containing
the English definitions. These definitions are then automatically
imported by the locale file inheritance scheme.

Several new macros were added to allow for multi-lingual blocks:

 * `\thesis@authorEn`, `\thesis@advisor` -- They gets redefined
   via the `author` and `advisor` `\thesissetup` keys like their
   locale-agnostic counterparts, but the initial values are
   `\thesis@placeholders@authorEn` and
   `\thesis@placeholders@advisorEn` rather than
   `\thesis@placeholders@author` and
   `\thesis@placeholders@advisor`.
 * `\thesis@field` -- The field of study, required by the sci style
 * `\thesis@departmentEn`, `\thesis@programmeEn`, `\thesis@fieldEn`
   `\thesis@titleEn`, `\thesis@TeXtitleEn` -- The English versions
   of their locale-agnostic counterparts
 * `\thesis@abstractEn`, `\thesis@keywordsEn` -- These no longer
   default to `\undefined`, but instead use the fresh-defined
   `\thesis@placeholders@abstractEn` and
   `\thesis@placeholders@keywordsEn` locale macros.
 * `\thesis@date`, `\thesis@year`, `\thesis@month`, `\thesis@day`,
   `\thesis@season`, `\thesis@academicYear`, `\thesis@semester` --
   These are all parsed out of a single value assigned to the
   `date` key via `\thesissetup`.