reduce ram required by api amdmts (closes #109)
reduce ram required by api amdmts (closes #109)

Warning @mdamien: this will break the-law-factory-parser as the format returned for amendment's "groupes_parlementaires" changes

Tested on :
- https://www.nosdeputes.fr/15/amendements/485/json
- https://www.nosdeputes.fr/15/amendements/714/json
- https://www.nosdeputes.fr/15/amendements/857/json

All pass with the regular 65M ram allowed to php now

Benchmarks of min ram required with various experiments:
                         485  714  857
old                       75   85  105
-slugs                    65   75   95
-slugs light-gpes         45   65   85
-slugs lighter-gpes       35   35   35
light-slugs lighter-gpes  35   35   65

Even lighter slugs are heavy but it passes under the regular limit so I prefered keeping them for consistancy

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