Better polygon validation

Possible follow-up to #163 (closed): we could use ST_IsValid or ST_IsRing to further validate zone polygons and avoid weird polygons that simply have no reason to exist. The following polygons are considered invalid:

Invalid polygons in preprod

At the time of writing this issue, there are 2838 invalid polygons in preprod, out of 10554485 zones:

• 999 polygons from Tobacco800 are supposed to be rectangles, like those ones, but actually are "bowties".
  Those have been fixed manually in a Python script, by moving one point in the polygon. The same issue occurred in production and was fixed in the past.
• One polygon is some dumb test of the annotation interface I did a while ago.
• 1838 polygons are related to transcriptions from Valencià and Transkribus.

Invalid polygons in prod

Elements

Corpus	Count
Total	796
PRImA	427
RDCL2019	131
cBAD 2019	130
HORAE | DLA Annotations	70
Bozen	15
BNPP-archives	8
AN-Index	6
READ-BAD2017	5
BALSAC | Balsac annotations	2
Test Martin	1
CaptchAN	1

SQL query

select coalesce(c.name, 'Total'), count(*)
from images_zone z
inner join documents_element e on (e.zone_id = z.id)
inner join documents_corpus c on (c.id = e.corpus_id)
where not ST_IsRing(z.polygon)
group by rollup(c.name)
order by count(*) desc;

Transcriptions

Source	Count
Total	1305563
litis_lexical_ocr	653072
litis_raw_ocr	649828
manual	2550
transkribus	108
kaldi_bnpp	2
kaldi_bnpp_03	1
kaldi_bnpp_04	1
kaldi_bnpp_oscar	1

SQL query

select coalesce(ds.slug, 'Total'), count(*)
from images_zone z
inner join documents_transcription t on (t.zone_id = z.id)
inner join documents_datasource ds on (ds.id = t.source_id)
where not ST_IsRing(z.polygon)
group by rollup(ds.slug)
order by count(*) desc;