Add cls.LT and cls.EXACT to models.DateTimeField

Closes: #207 - allow queries to identify a range of test
jobs between specific dates.

Signed-off-by: Neil Williams <neil.williams@linaro.org>
13 jobs from datetime in 11 minutes and 53 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #26327
amd64
dispatcher-debian-10

00:03:25

passed #26325
amd64
dispatcher-debian-9

00:03:39

passed #26328
amd64
server-debian-10

00:02:51

passed #26326
amd64
server-debian-9

00:03:15

 
  Analyze
passed #26329
amd64
black

00:00:59

passed #26331
amd64
code_quality

00:00:26

passed #26330
amd64
codestyle

00:00:47

passed #26332
dind amd64 allowed to fail
sast

00:02:05

 
  Build
passed #26335
amd64
amd64/pkg-debian-10

00:02:36

passed #26334
amd64
amd64/pkg-debian-9

00:02:22

passed #26333
amd64
doc

00:02:29

 
  Docker
passed #26336
dind amd64
amd64/lava-dispatcher

00:01:42

passed #26337
dind amd64
amd64/lava-server

00:03:01