Fix layout for the reviews sheet

Summary:
* the main contentitem of a sheet should have *always* an explicit implicitWidth
* never use anchors for delegates

Test Plan:
ireviews sheet works ok now both in desktop and mobile modes
before:
{F7875527}
after:
{F7875529}

Reviewers: #plasma, #vdg, ngraham

Reviewed By: #vdg, ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26566