Bug 34959: Sort PO files correctly

Bug 34959: Sort PO files correctly

The PO files are not sorted when we update them which leads to
unnecessary changes that are commited: hard to see differences and
make git index grow superfluously.

Test plan:
0. Do not apply this patch
1. gulp po:update --lang es-ES
2. git commit -a -m"First PO update"
3. Run again the gulp update command
4. git diff
=> You have a lot of changes generated here, the po:update is not
idempotent.

5. Apply this patch
6. Run the gulp update command
7 git commit -a -m"PO update after 34959"
8. Run the gulp update command
9. git diff
=> No changes are generated

Note that this patch will all the entries by files, and per line
numbers.
It fixes a bug in some condition, where we add information/context about
the string. For instance search for "For the first occurrence" in the
file. Prior to this patch this was not correct, we didn't add info about
the first occurrence (but whichever in the list).

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit 4b337b107e16d47ab79571b9a38d273c9e980b20)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>