Make the size of a browse result page configurable.

[mwoodiupui]

Description

Invent a new configuration property to control the number of results per page when displaying browse results.

Instructions for Reviewers

List of changes in this PR:

• Create a new configuration property browseBy.pageSize.
• Define a default value wherever browseBy is being configured.
• Replace hard-coded "20" with the value of this property in the browse-by-metadata component.

Checklist

• [x] My PR is small in size (e.g. less than 1,000 lines of code, not including comments & specs/tests), or I have provided reasons as to why that's not possible.
• [x] My PR passes TSLint validation using yarn run lint
• [x] My PR doesn't introduce circular dependencies
• [x] My PR includes TypeDoc comments for all new (or modified) public methods and classes. It also includes TypeDoc for large or complex private methods.
• [x] My PR passes all specs/tests and includes new/updated specs or tests based on the Code Testing Guide.
• [x] If my PR includes new, third-party dependencies (in package.json), I've made sure their licenses align with the DSpace BSD License based on the Licensing of Contributions documentation.