Add settings to control padding between activities on a course page

[Manoj Solanki]

Following on from: https://moodle.org/mod/forum/discuss.php?d=364711

Add a setting for the amount of padding (in px) between activities. Default will need to be set to what is present currently in the CSS (so as not to affect existing installations that have got used to the padding). Possibly add one setting each for bottom and top padding.

[Manoj Solanki]

Fix #883. Remove excess padding and margin spacing between activities. Add setting to control top and bottom margin spacing for users that prefer to have more space.

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