More complete editing for existing shares

**is your feature request related to a problem? Please describe.**

This happened to me just now:
- create share set to 1d expiration
- realize "actually I want to set to to 1 week"
- go to ?shares
- the only buttons are for +1m or +1hr, and I didn't really want to click `1hr` 168 times

I popped open the console and edited in the button I wanted, but it would be nice if there was a proper way.

**Describe the idea / solution you'd like**

An edit screen that has all the same fields as the screen for creating a share:
- name
- source
- password
- expiry (with input for days)
- perms

**Describe any alternatives you've considered**

deleting and re-creating the share: totally reasonable workaround, but a proper edit function would be nice

查看 frontend 中的 share 管理 UI（可能位于 HTML/JS 模板中）以及 backend 的 share 处理逻辑（Python）。编辑页面应与创建 share 表单保持一致。检查现有的 share 创建和管理路由，以了解数据模型。实现一个编辑 endpoint，并更新 UI，加入一个编辑按钮，将其链接到包含 name、source、password、expiry（带 days 输入）的表单。

由索引模型根据 Issue 内容生成。