Configurable options for the "Share" option

## Issue

Currently the "Share" option is hardcoded (in `/main/ui/src/components/Share/index.tsx`) to offer options of link copy, facebook, twitter, or system share. This seems very inflexible, and a number of use cases could all benefit from having the share options being configurable.

## Use cases

1. Corporate intranet deployments of Answer, where sharing to external sites is not allowable or should be discouraged. In these use cases, removal of some/all sharing options might be preferred.
2. Changes in the sharing landscape. For example, "Twitter" is now "X", and that shouldn't require a software rebuild to accommodate.
3. Support for new options. For example, Adding BlueSky, or other new / up and coming sharing sites.

## Suggested solution

Make the sharing options (names and endpoints) configurable in either the Admin -> Settings or Admin -> Customize panels. Might require a two step solution, where one step is configuring the name and endpoint, and the other step is the ability to define a new / non-standard endpoint that can be referenced.

Start with /main/ui/src/components/Share/index.tsx, then inspect the Admin → Settings and Admin → Customize panels mentioned in the issue to understand where configurable options could belong. Done means share names and endpoints can be configured, existing options can be removed or changed, and new endpoints can be supported without a software rebuild.

Written by the indexing model from the issue text.