settings: Add default for INLINE_URL_EMBED_PREVIEW.
settings: Add default for INLINE_URL_EMBED_PREVIEW.

Tweaked by tabbott to improve the documentation around this setting
and its INLINE_IMAGE_PREVIEW cousin.

kou/zulip,brainwane/zulip,jackrzhang/zulip,verma-varsha/zulip,synicalsyntax/zulip,andersk/zulip,brockwhittaker/zulip,vaidap/zulip,rishig/zulip,jackrzhang/zulip,hackerkid/zulip,eeshangarg/zulip,mahim97/zulip,jackrzhang/zulip,rht/zulip,tommyip/zulip,zulip/zulip,hackerkid/zulip,verma-varsha/zulip,andersk/zulip,timabbott/zulip,vaidap/zulip,eeshangarg/zulip,dhcrzf/zulip,tommyip/zulip,mahim97/zulip,synicalsyntax/zulip,dhcrzf/zulip,brockwhittaker/zulip,eeshangarg/zulip,synicalsyntax/zulip,jackrzhang/zulip,brockwhittaker/zulip,mahim97/zulip,timabbott/zulip,vaidap/zulip,punchagan/zulip,zulip/zulip,hackerkid/zulip,showell/zulip,zulip/zulip,dhcrzf/zulip,andersk/zulip,jackrzhang/zulip,rht/zulip,rishig/zulip,zulip/zulip,zulip/zulip,synicalsyntax/zulip,vaidap/zulip,brockwhittaker/zulip,Galexrt/zulip,kou/zulip,brockwhittaker/zulip,shubhamdhama/zulip,kou/zulip,Galexrt/zulip,timabbott/zulip,tommyip/zulip,showell/zulip,punchagan/zulip,eeshangarg/zulip,kou/zulip,jackrzhang/zulip,tommyip/zulip,synicalsyntax/zulip,timabbott/zulip,eeshangarg/zulip,rht/zulip,amanharitsh123/zulip,showell/zulip,hackerkid/zulip,shubhamdhama/zulip,verma-varsha/zulip,punchagan/zulip,brainwane/zulip,vaidap/zulip,rht/zulip,rishig/zulip,shubhamdhama/zulip,showell/zulip,amanharitsh123/zulip,andersk/zulip,hackerkid/zulip,punchagan/zulip,rishig/zulip,dhcrzf/zulip,brainwane/zulip,jackrzhang/zulip,rht/zulip,hackerkid/zulip,Galexrt/zulip,zulip/zulip,zulip/zulip,vaidap/zulip,amanharitsh123/zulip,brainwane/zulip,brainwane/zulip,kou/zulip,shubhamdhama/zulip,synicalsyntax/zulip,eeshangarg/zulip,mahim97/zulip,kou/zulip,andersk/zulip,rishig/zulip,dhcrzf/zulip,eeshangarg/zulip,punchagan/zulip,amanharitsh123/zulip,mahim97/zulip,dhcrzf/zulip,timabbott/zulip,shubhamdhama/zulip,amanharitsh123/zulip,verma-varsha/zulip,shubhamdhama/zulip,rishig/zulip,andersk/zulip,showell/zulip,synicalsyntax/zulip,verma-varsha/zulip,amanharitsh123/zulip,Galexrt/zulip,timabbott/zulip,Galexrt/zulip,verma-varsha/zulip,Galexrt/zulip,showell/zulip,andersk/zulip,punchagan/zulip,tommyip/zulip,dhcrzf/zulip,brainwane/zulip,kou/zulip,punchagan/zulip,rht/zulip,Galexrt/zulip,hackerkid/zulip,brainwane/zulip,rishig/zulip,tommyip/zulip,shubhamdhama/zulip,timabbott/zulip,tommyip/zulip,brockwhittaker/zulip,showell/zulip,rht/zulip,mahim97/zulip