Add body length check for Django's capture_body

[basepi]

What does this pull request do?

We had no restrictions on the body length in Django. This could result in the event payload being rejected by the APM Server.

As an aside, we'll need to figure out a better solution in the long term. (#305)

Related issues

closes #1478

[apmmachine]

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• Start Time: 2022-08-15T18:13:30.459+0000

• Duration: 31 min 2 sec

Test stats :test_tube:

Test	Results
Failed	0
Passed	4969
Skipped	3420
Total	8389

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Tests succeeded.

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[apmmachine]

:globe_with_meridians: Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (68/68)	:green_heart:
Files	100.0% (227/227)	:green_heart:
Classes	100.0% (227/227)	:green_heart:
Lines	89.751% (17444/19436)	:+1: 0.008
Conditionals	77.18% (3186/4128)	:+1: 0.022

[basepi]

/test