Serve assets from issuer account

[charlie-wasp]

What we've got here:

• I ditched Horizon regarding assets, and now we're getting assets directly from the database, namely from special postgres view, made by @nebolsin. I expanded it a little, I added flags and appended the row for the native XLM to serve everything consistently
• Now we have the connection to assets from an account, so user now can query all assets, issued by the particular account in a graph-like manner
• thanks to @nebolsin, we now can serve more data about an asset, like holders count and overall tokens number in circulation
• authorization flags now live under assets, not under accounts

More technical notes:

• I dropped AssetWithInfo from the schema, we have one type Asset now
• I decided to omit native XLM from query { assets } response because it breaks alphanumeric sorting in pagination. It happens because we use "native" as an id for XLM, which serves as pagination token too, so native XLM won't be between other assets with codes, starting on "X". And it seems not very useful to have it there anyway. If the user would like to obtain some statistics about lumens, they can do it via query { asset(id: "native") }
• It would be better to use the Asset class from stellar-sdk only for parsing XDR, and use our new Asset class everywhere else, but I didn't refactor that for now, because it looks like a big deal. Definitely, something to do later

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This change is 

[charlie-wasp]

Also here is the new assets view definition:

( SELECT (t.assetcode::text || '-'::text) || t.issuer::text AS assetid,
    t.assetcode AS code,
    t.issuer,
    sum(t.balance) AS total_supply,
    sum(t.balance) FILTER (WHERE t.flags = 1) AS circulating_supply,
    count(t.accountid) AS holders_count,
    count(t.accountid) FILTER (WHERE t.flags = 0) AS unauthorized_holders_count,
    ( SELECT a.flags
           FROM accounts a
          WHERE t.issuer::text = a.accountid::text) AS flags,
    max(t.lastmodified) AS last_activity
   FROM trustlines t
  GROUP BY t.issuer, t.assetcode
  ORDER BY (count(t.accountid)) DESC)
UNION
 SELECT 'native'::text AS assetid,
    'XLM'::character varying AS code,
    NULL::character varying AS issuer,
    sum(accounts.balance) AS total_supply,
    sum(accounts.balance) AS circulating_supply,
    count(*) AS holders_count,
    0 AS unauthorized_holders_count,
    0 AS flags,
    max(accounts.lastmodified) AS last_activity
   FROM accounts;

[nebolsin]

Reviewed 12 of 17 files at r1, 18 of 18 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved