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.

/main/ui/src/components/Share/index.tsx から始め、次に issue で言及されている Admin → Settings パネルと Admin → Customize パネルを調べて、設定可能なオプションをどこに配置できるかを理解します。Share の名前とエンドポイントを設定でき、既存のオプションを削除または変更でき、新しいエンドポイントをソフトウェアの再ビルドなしでサポートできれば完了です。

索引モデルが issue の本文から書いたものです。