Allow keeping proxy tags when proxying messages.

Required database migration:

Closes #75.

PluralKit.Core/Models.cs

@@ -66,6 +66,7 @@ namespace PluralKit
         [JsonProperty("pronouns")] public string Pronouns { get; set; }
         [JsonProperty("description")] public string Description { get; set; }
         [JsonProperty("proxy_tags")] public ICollection<ProxyTag> ProxyTags { get; set; }
+        [JsonProperty("keep_proxy")] public bool KeepProxy { get; set; }
         [JsonProperty("created")] public Instant Created { get; set; }
         
         // These are deprecated as fuck, and are kinda hacky

PluralKit.Core/Stores.cs

@@ -495,7 +495,7 @@ namespace PluralKit {
 
         public async Task SaveMember(PKMember member) {
             using (var conn = await _conn.Obtain())
-                await conn.ExecuteAsync("update members set name = @Name, display_name = @DisplayName, description = @Description, color = @Color, avatar_url = @AvatarUrl, birthday = @Birthday, pronouns = @Pronouns, proxy_tags = @ProxyTags where id = @Id", member);
+                await conn.ExecuteAsync("update members set name = @Name, display_name = @DisplayName, description = @Description, color = @Color, avatar_url = @AvatarUrl, birthday = @Birthday, pronouns = @Pronouns, proxy_tags = @ProxyTags, keep_proxy = @KeepProxy where id = @Id", member);
 
             _logger.Information("Updated member {@Member}", member);
         }

PluralKit.Core/db_schema.sql

@@ -32,7 +32,8 @@ create table if not exists members
     birthday     date,
     pronouns     text,
     description  text,
-    proxy_tags   proxy_tag[]    not null default array[], -- Rationale on making this an array rather than a separate table - we never need to query them individually, only access them as part of a selected Member struct 
+    proxy_tags   proxy_tag[]    not null default array[], -- Rationale on making this an array rather than a separate table - we never need to query them individually, only access them as part of a selected Member struct
+    keep_proxy   bool           not null default false, 
     created      timestamp      not null default (current_timestamp at time zone 'utc')
 );